ILF 


TRIAL     CHEM  ISTKY 


BEING    A    SERIES    OF    VOLUMES    GIVING 
A     COMPREHENSIVE     SURVEY    OK 

THE    CHEMICAL    INDUSTRIES 


BIOLOGY 
LIBRARY 


INDUSTRIAL    CHEMISTRY 

BEING  A   SERIES  OF  VOLUMES   GIVING  A 
COMPREHENSIVE   SURVEY   OF 

THE    CHEMICAL    INDUSTRIES 

EDITED  BY  SAMUEL  RIDEAL,  D.Sc.  LOND.,  F.I.C. 

FELLOW  OF  UNIVERSITY  COLLEGE,   LONDON 


ASSISTED   BY 


JAMES  A.  AUDLEY,  B.Sc.,  F.I.C. 
W.  BACON,  B.Sc.,  F.I.C.,  F.C.S. 

E.  DE  BARRY  BARNETT,  B.Sc.,  A.I.C. 
M.  BARROWCLIFF,  M.B.E.,  F.I.C. 
H.  GARNER  BENNETT,  M.Sc. 

F.  H.  CARR,  C.B.E.,  F.I.C. 

S.  HOARE   COLLINS,  M.Sc.,  F.I.C. 
H.   C.  GREENWOOD,   O.B.E.,  D.Sc., 

F.I.C. 
R.  S.  MORRELL,  M.A.,  PH.D. 


J.  R.  PARTINGTON,  M.A.,  PH.D. 
ARTHURS.  PRATT.B.Sc.,  Assoc.R.S.M. 
ERIC  K.  RIDEAL,  M.B.E.,  D.Sc.,  M.A. 

PH.D.,  F.I.C. 

W.  H.  SIMMONS,  B.Sc.,  F.I.C. 
R.  W.  SINDALL,  F.C.S. 
HUGH  S.  TAYLOR,  D.Sc. 
ARMAND  DE  WAELE,  B.Sc. 
C.  M.  WHITTAKER,  B.Sc. 
&c.,  &c. 


SILICA    AND    THE 
SILICATES 


BY 


JAMES   A.   AUDLEY,  B.Sc.Lond.,  F.I.C. 

CERAMIC   CHEMIST 


NEW   YORK 
D.    VAN     NOSTRAND    COMPANY 

EIGHT   WARREN   STREET 
1921 


PRINTED   IN  GREAT  BRITAIN 

BIOLOGY 
LIBRARY 


GENERAL    PREFACE 

THE  rapid  development  of  Applied  Chemistry  in  recent  years 
has  brought  about  a  revolution  in  all  branches  of  technology. 
This  growth  has  been  accelerated  during  the  war,  and  the 
British  Empire  has  now  an  opportunity  of  increasing  its 
industrial  output  by  the  application  of  this  knowledge  to  the 
raw  materials  available  in  the  different  parts  of  the  world. 
The  subject  in  this  series  of  handbooks  will  be  treated  from 
the  chemical  rather  than  the  engineering  standpoint.  The 
industrial  aspect  will  also  be  more  prominent  than  that  of 
the  laboratory.  Bach  volume  will  be  complete  in  itself,  and 
will  give  a  general  survey  of  the  industry,  showing  how 
chemical  principles  have  been  applied  and  have  affected 
manufacture.  The  influence  of  new  inventions  on  the 
development  of  the  industry  will  be  shown,  as  also  the 
effect  of  industrial  requirements  in  stimulating  invention. 
Historical  notes  will  be  a  feature  in  dealing  with  the 
different  branches  of  the  subject,  but  they  will  be  kept 
within  moderate  limits.  Present  tendencies  and  possible 
future  developments  will  have  attention,  and  some  space 
will  be  devoted  to  a  comparison  of  industrial  methods  and 
progress  in  the  chief  producing  countries.  There  will  be  a 
general  bibliography,  and  also  a  select  bibliography  to  follow 
each  section.  Statistical  information  will  only  be  introduced 
in  so  far  as  it  serves  to  illustrate  the  line  of  argument. 

Each  book  will  be  divided  into  sections  instead  of 
chapters,  and  the  sections  will  deal  with  separate  branches 
of  the  subject  in  the  manner  of  a  special  article  or  mono- 
graph. An  attempt  will,  in  fact,  be  made  to  get  away  from 


vi  GENERAL  PREFACE 

the  orthodox  textbook  manner,  not  only  to  make  the  treat- 
ment original,  but  also  to  appeal  to  the  very  large  class  of 
readers  already  possessing  good  textbooks,  of  which  there 
are  quite  sufficient.  The  books  should  also  be  found  useful 
by  men  of  affairs  having  no  special  technical  knowledge,  but 
who  may  require  from  time  to  time  to  refer  to  technical 
matters  in  a  book  of  moderate  compass,  with  references  to 
the  large  standard  works  for  fuller  details  on  special  points 
if  required. 

To  the  advanced  student  the  books  should  be  especially 
valuable.  His  mind  is  often  crammed  with  the  hard  facts 
and  details  of  his  subject  which  crowd  out  the  power  of 
realizing  the  industry  as  a  whole.  These  books  are  intended 
to  remedy  such  a  state  of  affairs.  While  recapitulating  the 
essential  basic  facts,  they  will  aim  at  presenting  the  reality 
of  the  living  industry.  It  has  long  been  a  drawback  of  our 
technical  education  that  the  college  graduate,  on  commencing 
his  industrial  career,  is  positively  handicapped  by  his 
academic  knowledge  because  of  his  lack  of  information  on 
current  industrial  conditions.  A  book  giving  a  compre- 
hensive survey  of  the  industry  can  be  of  very  material 
assistance  to  the  student  as  an  adjunct  to  his  ordinary  text- 
books, and  this  is  one  of  the  chief  objects  of  the  present 
series.  Those  actually  engaged  in  the  industry  who  have 
specialized  in  rather  narrow  limits  will  probably  find  these 
books  more  readable  than  the  larger  textbooks  when  they 
wish  to  refresh  their  memories  in  regard  to  branches  of  the 
subject  with  which  they  are  not  immediately  concerned. 

The  volume  will  also  serve  as  a  guide  to  the  standard 
literature  of  the  subject,  and  prove  of  value  to  the  con- 
sultant, so  that,  having  obtained  a  comprehensive  view  of 
the  whole  industry,  he  can  go  at  once  to  the  proper 
authorities  for  more  elaborate  information  on  special  points, 
and  thus  save  a  couple  of  days  spent  in  hunting  through  the 
libraries  of  scientific  societies. 

As  far  as  this  country  is  concerned,  it  is  believed  that 
the  general  scheme  of  tliis  series  of  handbooks  is  unique, 
and  it  is  confidently  hoped  that  it  will  supply  mental 


GENERAL   PREFACE  vii 

munitions  for  the  coming  industrial  war.  I  have  been 
fortunate  in  securing  writers  for  the  different  volumes  who 
are  specially  connected  with  the  several  departments  of 
Industrial  Chemistry,  and  trust  that  the  whole  series  will 
contribute  to  the  further  development  of  applied  chemistry 
throughout  the  Empire. 

SAMUEL   RIDEAI,. 


AUTHOR'S    PREFACE 

AN  apology  is  due  to  the  public,  the  publishers,  and  the 
general  editor,  for  the  delay  in  the  appearance  of  this  volume, 
but  it  is  hoped  that  the  additional  notices  of  recent  work 
(which  it  was  in  consequence  possible  to  include)  may  in 
some  measure  compensate  for  the  extra  time  of  waiting. 
When  the  work  was  undertaken,  it  was  realized  that  the 
task  in  hand  was  no  light  one,  but  its  magnitude  far  exceeded 
all  expectations.  The  vast  amount  of  material  which  was 
gradually  accumulated  rendered  it  necessary  to  consider 
carefully  whether  (in  order  to  keep  the  book  within  moderate 
limits)  every  section  should  be  treated  in  little  more  than 
bald  outline,  so  as  to  deal  in  a  more  or  less  uniform  fashion 
with  the  various  branches  falling  within  the  wide  scope  of 
the  general  scheme,  or  whether  it  might  not  be  more  useful 
to  deal  very  briefly  indeed  with  some  portions  so  that  more 
space  could  be  devoted  to  certain  selected  subjects  of  special 
interest.  It  was  eventually  decided  to  adopt  the  latter 
alternative,  though  it  was  inevitable  that  the  selection  should 
be  arbitrary,  having  regard  more  particularly  to  the  great 
amount  of  research  work  accomplished  during  the  last  few 
years,  especially  in  connection  with  glass  and  refractory 
materials.  Section  IV.  (Ceramic  Industries)  presented  an 
exceptionally  difficult  problem  in  this  connection.  Clay 
refractories  and  silica  refractories  have  received  special 
consideration,  and  other  refractories  have  been  dealt  with 
briefly  in  view  of  their  importance,  though  they  do  not 
strictly  fall  within  the  scope  of  the  treatise. 

No  previous  English  author  appears  to  have  attempted 
to  cover  the  same  range  of  subjects  within  the  limits  of  a 
single  volume,  and  in  the  French  treatise  of  Le  Chatelier 


x  AUTHOR'S  PREFACE 

bearing  the  same  title,  and  the  German  book  of  similar 
scope  by  Drs.  W.  and  D.  Asch,  the  treatment  is  on  entirely 
diff erect  lines,  though  the  former  furnished  many  valuable 
hints  and  suggestions.  Those  who  seek  fuller  information 
on  specific  topics  than  is  contained  in  the  present  volume 
will,  it  is  confidently  anticipated,  find  ample  references  to 
the  best  available  literature.  At  the  same  time,  it  is  hoped 
that  this  book  in  itself  may  have  some  value  in  meeting  a 
want,  quite  apart  from  such  references.  After  due  delibera- 
tion, it  was  decided  to  select  illustrations  as  far  as  practicable 
from  apparatus  and  appliances  actually  put  on  the  market, 
so  that  reliance  might  be  placed  on  their  suitability  for 
their  purpose,  and  thanks  are  due  to  the  various  firms 
who  have  supplied  them.  It  does  not  necessarily  follow 
that  any  particular  contrivance  is  adapted  only  for  the 
purpose  for  which  it  happens  to  be  used  as  an  illustration. 
Thus,  grinding  apparatus  of  different  types  will  be  found 
illustrated  in  connection  with  the  grinding  of  cement,  but 
most  (if  not  all)  would  be  equally  available  for  grinding 
other  materials  of  approximately  the  same  degree  of  hardness. 

Every  care  has  been  taken  to  avoid  inaccurate  or 
ambiguous  statements,  and  the  hope  is  entertained  that 
as  far  as  it  goes  the  book  may  provide  a  thoroughly  reliable 
guide  to  the  important  industries  concerned. 

The  Author  desires  to  acknowledge  valuable  suggestions 
and  references  supplied  by  the  general  editor. 


JAMBS  A. 


HANLEY, 

February,  1921. 


CONTENTS 


PAGE 

GENERAL  PREFACE   v 

AUTHOR'S  PREFACE .       ix 

CONTENTS  xi 


SECTION    I.— SILICA. 

Silicon.    Silica  as  a  constituent  of  the  earth's  crust.     Varieties  of  silica,  and 
the  uses  of  some  of  them.     Quartz  and  its  varieties.     Tridymite,  Opal, 

Flint,  Chalcedony,  Agate,  etc. i 

Quartzite.     Sands.     Sandstones     .          .          .          .          .          .          .          .26 

Diatomaceous  earth,  or  Kieselguhr 41 

Fused  silica  and  its  applications     .......  42 

Literature    .         .         .         .         .         .         .         .  .         .         -45 


SECTION    II.— SILICATES. 

Some  natural  and  artificial  silicates.  Silicates  of  alumina,  anhydrous  and 
hydra  ted  (including  kaolin,  clays,  fuller's  earth,  etc.).  Shales  and 
Slates.  Felspars,  Micas,  Chlorite,  Talc,  and  Steatite.  Serpentine, 
Meerschaum,  Leucite,  Lapis-Lazuli,  Zeolites,  artificial  Zeolite,  or  Per- 
mutite.  Water-glass.  Hornblende,  Augite,  and  Asbestos.  Enstatite, 
Wollastonite,  Garnets,  Idocrase  (Vesuvianite),  Epidote,  Zircon, 
Topaz,  Emerald  and  Beryl,  Tourmaline  and  Schorl,  Olivine  .  .  46 
Granites  and  other  igneous  rocks.  Cornish  Stone  (China  Stone)  .  .  102 

Lavas,  Volcanic  tufas,  Obsidian,  Pumice 104 

Slags 109 

Ferrosilicon  and  other  metallic  products  containing  silicon  as  an  essential 

constituent 112 

Carborundum  and  allied  substances 115 

Literature    .  122 


SECTION  III.— LIME,  CEMENT,  AND  MORTAR. 

Materials  used.    Lime,  its  preparation  and  properties       .         .         .         .123 
Plaster  of  Paris,  its  preparation,  properties,  and  uses          .         .         .         .133 
Common  mortar.     Hydraulic  lime  and  mortar,     Roman  cement.     Portland 
cement.     Burning  and  grinding  of  cement.     Constitution  of  Portland 
cement.     Setting  and   hardening    of   cement.     Puzzuolana    cements. 
Concrete.     Mixed  cements.     Some  special  cements    .         .         .         .     135 

Literature  ,  167 

zi 


CONTENTS 
SECTION   IV.— CERAMIC   INDUSTRIES. 

PACE 

Meaning  of  the  word  *'  ceramic. "    Clay  the  basis  of  nearly  all  ceramic  wares. 
General  and  special  characters  of  clays  for  ceramic  purposes.     Cornish 
Stone  and  Felspar  ..........     168 

Varieties  of  pottery  and  porcelain.     Processes  in  the  preparation  of  earth- 
enware, including  grinding  and  mixing,   drying,  kneading,  shaping, 
drying,  firing,  glazing,  and  decoration.     Stoneware.    English  porcelain, 
or  china.     Parian.     French  soft  porcelain.     Hard   porcelain.     Clay 
tobacco  pipes.     Common  pottery.     Terra-cotta.    Tiles.    Bricks.     Fire- 
bricks.    Crucibles.     Fireclay  ware.     Silica  bricks     .         .         .         .178 

Other  refractory   materials  (magnesite,  dolomite,   chromite,  alumina,  silli- 

manite,  zirconia,  zircon) .........     266 

British  developments    ....  .270 

Trade  statistics    ...  .  .271 

Literature    ...  ...  .272 

SECTION   V.— GLASS  AND   ENAMELS. 

Glassmaking  an  ancient  industry.  Composition  and  properties  of  glass. 
Raw  materials.  Mixing.  Decolorizing.  Glass  furnaces.  Glasshouse 
pots.  Tanks.  Principal  types  of  glass.  Soda-lime  glass  (crown  glass, 
sheet  glass,  plate  glass,  bottle  glass).  Flint  glass  or  crystal.  Devitri- 
fication. Some  special  kinds  of  glass.  Aventurine  glass.  Gold  ruby 
glass.  Red  glass,  etc.  Glass  painting  or  staining.  Optical  glass. 

Strass  and  imitation  gems 273 

Enamels      . 

British  developments     ...  •  ...     332 

Trade  statistics     ...  ...  -334 

Literature *         '         '         *     334 

SECTION   VI.— MISCELLANEOUS   APPLICA- 
TIONS  OF  SILICATES. 

Special  products  from  clay,  slates,  and  shales,  and  from  felspar  and  slag. 

Alum,  aluminium  sulphate,  slag,  fertilizers,  soils         ....  33^ 

Earthy  and  other  silicate  pigments.     Ochres,  siennas,  umbers,  etc.     .         .  350 

Literature    .  .'.... 357 

Additional  Literature  .         .        V 35$ 

INDEX  ...  ....    361 


LIST   OF   ILLUSTRATIONS 

FIG.  NO.  PAGE 

1.  Crusher  and  pulverizer   .         .        .         .         .         .         .  .  .  15 

2.  The  Bradley  three-roll  mill    .         .         .         .         .         .  .  .140 

3.  Hecla  disc  crusher  .         .;       *.      . •   •'    f         .         .  .  .  142 

4.  Allen's  stag  mill .  .  .  143 

5.  ,,        „        ,,     sectional  view     .         .         .         .         .  .  .  143 

6.  Sturtevant  open-door  ring  roll-mill  .         ,         .         .  .^      >  .  144 

7»  »                 »              »          »  •         •         «         •       ,  •  ,    •  •  *44 

8.  The  Griffin  mill    .         .         .  .         .        f.  :      .  .  146 

9.  Rotary  cement  kiln        .         .        ..         .         *         .         .  .  .  148 

10.  Ball-mill  for  dry  grinding       ,         .         .         .         .         .  »•'   '     .  151 

11.  Pulverizing-mill  with  solid  bottom  .         .          .  .  .  181 

12.  Over-geared  slip  blunger ,  .  .  184 

13.  Patent  automatic  deadweight  pump           ,         .         *     ,    .  .  .  187 

14.  Iron  filter  press .         .  .  .  188 

15.  Pug-mill .  .  189 

1 6.  Hand -power  throwing  wheel  .         .         .         .         ...  191 

17.  Power-driven  potter's  lathe    .         .         .         ,  .  ,  .  192 

18.  Rope-driven  jigger  and  jolley         v         .         .      .  .         .  .  .  197 

19.  Stiff  plastic  machine  for  brick-making      .         .  .  .  .  249 

20.  "  Staffordshire "  kiln     *        *         .         .         .         ...  .253 

21.  Dunnachie's  patent  continuous  regenerative  gas-fired  kiln  .  .  254 

22.  Boetius  furnace  (cross  section)         .         .         .         .         .  .  .  291 

23.  ,,  ,,        (longitudinal  section)      .         .         .         .  .  .  292 

24.  Hermansen  recuperative  furnace     .    '     .         .         .         .  .  .  293 

25.  Stein  recuperative  pot  furnace         .         .         .         .         .  v  .  294 

26.  Teisen  patent  gas-fired  Lehr  with  conveyor       .         .         ,  ,  .  296 

27.  Stein  and  Atkinson  tank  furnace     .  .-•...         .         .         .  .  .  301 


ERRATA 

Page  14,  line  19,  for  "and  in  glass  manufacture  for  cutting  glass,"  read  "and 
for  cutting  glass  in  its  manufacture." 

Page    118,    line    19,   for    "Powdered    crystalline    silicon    carbide,"    read 
"  Powdered  silicon  carbide." 

Page  146,  add,  after  inscription  to  Fig.  8,  "(Bradley  Pulverizer  Co.)." 
Page  152,  line  n,  for  "sieves,"  read  "sleeves." 

Page  153,  5  lines  from  bottom,  for  "consistent  with  age,"  read  "consistent 
with  increasing  age." 

Page  154,  line  28,  for  "  loss  of  ignition,"  read  "loss  on  ignition." 
Page  157,  line  26, for  "alumina-silicic"  read  " alumino-silicic. " 
Page  158,  line  25,  for  "  perfectly,"  read  "imperfectly." 
Page  172,  line  12,  for  "Coarser,"  read  "  Coarse." 


SILICA 
AND  THE  SILICATES 

SECTION  I.— SILICA 

SnjCA  is  an  oxide  of  the  non-metallic  element  silicon, 
which  is  itself  an  unfamiliar  substance,  though  its  compounds 
occur  very  abundantly  in  nature.  Silicon  is  never  found  in 
the  free  state.  It  is  known  in  three  modifications  : 

1.  Amorphous    Silicon,  first    obtained    by    Berzelius 
(1823)  by  heating  dry  silico-fluoride  of  potassium  or  sodium 
with   an   equal   weight   of   metallic   potassium    or  sodium 
respectively.    It  has  since  been  obtained  by  the  action  of 
heated  potassium,  sodium,  or  aluminium,  on  silicon  chloride 
vapour ;   by  the  action  of  heated  sodium. on  gaseous  silicon 
fluoride ;    by  heating  silica  with  magnesium   powder   (or 
with  aluminium),  mixed  with  magnesia  so  that  the  action 
may  not  proceed  too  violently ;  by  sparking  liquid  silicon 
hydride  ;  and  by  electrolysis  of  a  fused  mixture  of  potassium 
fluoride  and  potassium  silico-fluoride  ;  also  by  other  methods. 

Amorphous  silicon  is  a  brown  or  maroon-coloured 
powder,  with  a  specific  gravity  of  2*35  ;  it  readily  fuses  and 
volatilizes  in  the  electric  furnace,  dissolves  in  many  molten 
metals  (silver,  iron,  aluminium,  etc.),  readily  absorbs  gases  and 
water  vapour,  which  can  only  be  expelled  at  a  red  heat.  It  is 
attacked  by  hot  water  in  glass  vessels,  but  is  not  affected  by 
concentrated  hydrofluoric  acid  at  100°.  It  oxidizes  super- 
ficially in  air,  but  when  heated  readily  burns  to  form  silica. 

2.  Graphitoidal  Silicon. — This  was  obtained  by  Ber- 
zelius by  heating  amorphous  silicon  in  a  platinum  crucible. 
Other  methods  are  the  action  of  heat  on  a  mixture  of 

C,  I 


2  SILICA    AND  THE  SILICATES 

aluminium  with  potassium  silico-fluoride,  and  the  fusion 
with  aluminium  and  cryolite  of  the  product  obtained  by 
heating  magnesium  with  finely-powdered  sand  ;  on  treating 
the  first  of  these  products  with  hydrochloric  acid  and  then 
with  hydrofluoric  acid,  the  graphitoidal  silicon  is  left  in  the 
form  of  hexagonal  tables  with  curved  edges  ;  treatment  of 
the  second  product  with  water  and  acids  leaves  black 
glistening  spangles  of  graphitoidal  silicon.  Still  another 
method  is  by  fusing  aluminium  with  leadless  glass  and 
cryolite. 

Graphitoidal  silicon  is  not  readily  oxidized,  and  is  not 
attacked  by  acids  except  by  a  mixture  of  hydrofluoric  and 
nitric  acids,  though  it  is  soluble  in  strong  potash  or  soda. 
It  reduces  many  metallic  oxides. 

3.  Crystalline  or  Adamantine  Silicon. — This  is 
obtained  in  dark  steel-grey  globules,  sometimes  giving  double 
six-sided  pyramids,  by  heating  amorphous  silicon,  in  a 
platinum  crucible  lined  with  lime,  tq  a  temperature  between 
the  melting  points  of  steel  and  cast  iron.  It  is  also  obtained 
by  passing  silicon  chloride  vapour  over  fused  aluminium  in 
an  atmosphere  of  hydrogen ;  the  aluminium  becomes  saturated 
with  silicon,  and  afterwards  the  excess  of  silicon  separates 
in  large  dark  iron-grey  needles,  reddish  by  reflected  light  and 
iridescent.  The  form  is  derived  from  a  rhombic  octahedron, 
the  crystals  often  have  curved  faces,  and  they  can  cut  glass. 
Another  method  is  heating  to  redness  in  a  clay  crucible  a 
mixture  of  potassium  silico-fluoride  and  sodium  and  zinc  ; 
on  removing  the  zinc  as  far  as  possible  by  melting  out,  then 
treating  in  turn  with  hydrochloric  and  boiling  nitric  acid, 
the  crystalline  silica  remains  in  the  form  of  long  needles. 
The  most  convenient  method  of  preparation  is  by  heating 
aluminium  with  potassium  silico-fluoride,  and  purifying 
the  product. 

Crystalline  silicon  is  less  reactive  than  the  amorphous. 
If  finely  divided  it  is  attacked  by  boiling  water. 

Still  another  modification  of  silicon  was  obtained  by 
Moissan  and  Siemens,  along  with  ordinary  crystalline  silicon, 
by  dissolving  silicon  in  molten  silver.  This  modification 


SILICA  3 

differs  from  the  others  in  being  readily  attacked  by  hydro- 
fluoric acid.  Its  specific  gravity  is  2 '42. 

Silicon  has  received  no  important  industrial  application, 
but  I,e  Roy  recommends  it  for  use  in  electrical  heating  in 
place  of  metal  and  carbon  resistances.  The  resistance  of  a 
silicon  rod,  10  cms.  long,  and  of  40  sq.  mm.  cross  section, 
is  200  ohms,  as  compared  with  opi5  ohm  for  a  similar  carbon 
rod.  (i) 

Silica  as  a  Constituent  of  the  Earth's  Solid  Crust. 
— The  importance  of  silica  as  a  terrestrial  substance  may  be 
judged  from  the  fact  that  in  the  combined  and  uncombined 
condition  it  has  been  estimated  by  F.  W.  Clarke  to  form  close 
on  60  per  cent,  of  the  earth's  crust,  the  estimate  being  based 
on  careful  consideration  of  very  numerous  analyses  of  the 
solid  rocks  (including  loose  materials)  occurring  in  all  parts 
of  the  world,  and  calculated  to  a  depth  of  10  miles  below  the 
sea  level.  (5) 

As  might  be  expected,  much  of  this  material  is  available 
for  various  industrial  purposes. 

The  silica  sometimes  occurs  in  anhydrous  condition, 
and  sometimes  hydrated. 

Anhydrous  silica  presents  more  variations  than  any 
other  known  substance.  Some  varieties  are  crystalline, 
and  some  are  amorphous.  The  varieties  of  crystalline 
anhydrous  silica  fall  into  two  groups,  with  a  mean  specific 
gravity  of  about  2*6  and  about  2*3  respectively.  Bach  of 
these  varieties  when  heated  undergoes  reversible  trans- 
formations called  inversions,  giving  new  varieties  which  are 
stable  only  between  certain  limits  of  temperature. 

Silica  easily  assumes  the  vitreous  amorphous  state,  and 
communicates  it  to  mixtures  which  it  forms  with  different 
silicates  to  produce  ordinary  glasses ;  this  property  is 
possessed  in  so  high  a  degree  by  no  other  mineral  substance. 
Boric  acid  exists  well  in  the  vitreous  state,  but,  as  its  an- 
hydrous crystalline  state  is  unknown,  it  is  not  available  (like 
silica)  for  studying  the  passage  from  the  solid  amorphous 
state  to  the  crystalline  state. 

Hydrated  silica  shows  indefinite  variation  in  its  water 


4  SILICA   AND  THE  SILICATES 

content  with  the  condition  of  the  surrounding  medium.  It 
has  the  property  of  giving  with  water  so-called  colloidal 
solutions,  in  which  it  is  not  certain  whether  the  gelatinous 
masses  are  really  hydrated  chemically  or  are  rather  con- 
stituted of  extremely  fine  anhydrous  silica  forming  a  paste 
with  the  water. 

The  industrial  importance  of  silica  may  be  gathered  from 
the  extensive  use  of  silicious  materials  for  construction 
purposes.  Sand  for  mortar  is  nearly  pure  silica,  and  sand- 
stones have  been  largely  used  for  road  pavements  and  for 
buildings.  The  cathedral  of  Strassburg  is  an  important 
example  of  a  sandstone  structure. 

Many  silicates — in  some  cases  associated  with  silica 
itself — receive  useful  applications.  Granite,  being  very  re- 
sistant to  ordinary  atmospheric  influences,  is  used  for 
footways,  uprights  of  windows,  and  other  constructions. 
In  like  manner  porphyry,  petrosilex,  and  other  silicious 
materials,  including  trap  rocks,  make  good  road  metal. 
Slates  and  schists  are  used  for  roofing.  Volcanic  lavas, 
either  by  excavation  or  by  grinding  and  shaping,  followed 
by  heating  to  restore  the  condition  of  consolidation,  supply 
excellent  acid-proof  utensils  for  chemical  works.  Clay, 
shaped  and  burned,  gives  articles  much  used  for  construc- 
tion. Other  important  classes  of  silicates  provide  ceramic 
products,  glass  goods,  cements,  slags,  and  other  industrially- 
useful  materials. 

Anhydrous  Protoxide  of  Silicon  (SiO),  is  said  to  be 
obtained  as  a  chestnut-brown  powder,  very  light,  by  dis- 
tilling in  an  electric  furnace  a  mixture  of  silica  and  carbon 
(the  latter  being  insufficient  to  fully  reduce  the  silica),  and 
condensing  the  resulting  vapours  in  a  metallic  chamber  out 
of  contact  with  air.  The  specific  gravity  is  2*24 ;  it  is 
less  readily  soluble  in  hydrofluoric  acid  than  silica.  The 
powder  is  prepared  industrially  by  Potter's  process  (as  above, 
German  patent  189833,  Bng.  patent  1279/06),  and  sold  under 
the  name  of  Monox,  and  is  used  as  a  pigment  in  oil  painting. 
It  takes  a  much  greater  weight  of  oil  than  other  pigments — 
up  to  four  times  its  weight  as  compared  with  an  equal  weight 


SILICA  5 

for  light  pigments  like  ochres  and  one-fourth  the  weight  for 
heavy  pigments  like  minium  (red  lead).  The  composition 
of  the  brown  powder  approaches  that  of  the  protoxide, 
but  it  may  possibly  be  an  intimate  mixture  of  silicon  and 
silica,  (i)  and  (2) 

Silicones  and  Leucones  are  compounds  of  silicon  with 
oxygen  and  hydrogen,  having  properties  quite  different  from 
those  of  silica  and  silicon. 

Silicone  is  obtained  by  the  action  of  cold  concentrated 
hydrochloric  acid  on  a  calcium  silicide  rich  in  calcium.  It 
forms  transparent  yellow  lamellae,  which  bleach  in  light, 
giving  off  hydrogen,  and  becoming  transformed  into  leucone. 
Heated  in  a  vacuum,  it  is  completely  decomposed,  giving 
off  more  hydrogen,  and  leaving  a  brownish-black  mass, 
which  is  regarded  as  a  mixture  of  silicon  and  silica.  Heated 
in  contact  with  air,  it  inflames,  giving  silica,  slightly  coloured 
by  a  little  silicon.  It  is  not  attacked  by  cold  acids,  except 
hydrofluoric  acid,  which  dissolves  it  quickly.  Alkaline 
solutions  also  dissolve  it  quickly,  giving  off  hydrogen,  and 
forming  a  soluble  alkaline  silicate.  The  formula  of  silicone 
seems  to  be  Si2H2O2  or  Si2O.H2O,  according  to  Wohler, 
but  other  formulae  have  been  assigned  to  it. 

Leucones  are  white  substances  of  varied  composition, 
with  properties  quite  similar  to  those  of  silicone,  viz. 
transforming  into  silica  by  direct  oxidation,  dividing  into 
two  when  heated  not  in  contact  with  air,  liberating  hydrogen 
at  first,  then  silicon,  L,eucones  corresponding  to  SiO.H2O 
and  Si2O3.H2O  are  known,  their  properties  being  identical. 
Here,  also,  other  formulae  have  been  suggested. 

Several  other  compounds  of  silicon  with  hydrogen  and 
oxygen  are  known,  but  they  possess  only  theoretical  interest. 

Silica  exists  in  three  principal  varieties,  all  represented 
by  the  formula  SiO2  : 

i.  High  Specific  Gravity  Crystallized  Silica,  in- 
cluding quartz  and  chalcedony.  Quartz  has  specific  gravity 
2*65,  with  characteristic  crystalline  form.  Chalcedony  has 
specific  gravity  2*56,  and  occurs  in  compact  masses  with 
conchoidal  fracture  ;  its  microcrystalline  structure  is  only 


6  SILICA    AND  THE  SILICATES 

recognizable  by  examination  of   thin  sections  under  the 
polarizing  microscope. 

2.  Low    Specific    Gravity    Crystallized    Silica,    in- 
cluding tridymite  and  cristobalite.     Tridymite  has  specific 
gravity  2 '28,  and  it  is  found  as  very  small  crystalline  lamellae 
in  all  volcanic  rocks.     Cristobalite,  crystallized  in  pseudo- 
cubes,  and  having  specific  gravity  2*33,  is  of  very  rare  occur- 
rence,   in   certain   volcanic   soils   of   Mexico — it   was   first 
identified  in  specimens  from  San  Cristobal,  whence  its  name. 
It  is  formed  when  quartz  or  chalcedony  is  heated  to  a  high 
temperature   for    a   time   insufficient   to   produce   definite 
transformation  into  tridymite.     This  variety  is  especially 
characterized  by  extremely  irregular  expansion. 

3.  Amorphous  Silica,  including  vitreous  silica  and  the 
calcined  silica  of  laboratories.     Vitreous  silica  is  obtained 
by  fusion  of  any  other  variety  of  silica.     Calcined  silica 
results  from   dehydration   of  hydrated  silica  by  heating. 
It  is  a  white  powder,  and  very  light. 

4.  Hydrated   Silica   has   no    definite   composition   or 
crystalline    form.     It    consists    of    translucid,    gelatinous 
masses,  or  bundles  of  more  or  less  soft  clots,  which  have  none 
of  the  characters  of  ordinary  hydrated  acids.     Hydrated 
silica  is  prepared  by  the  action  of  an  acid  on  silicates  in  the 
presence    of    water.     The    liberated    silica    assumes    three 
different  forms.     On  pouring  slowly  a  solution  of  sodium 
silicate  into  concentrated  hydrochloric  acid,  no  precipitate 
is  formed,  the  silica  remaining  in  a  state  of  colloidal  solution. 
If  the  acid  be  poured  into  the  sodium  silicate  solution,  it 
forms  very  voluminous  gelatinous  clots  of  hydrated  silica. 
The  action  of  a  weaker  acid  like  formic  acid,  on  sodium 
silicate,  or  of  a  stronger  acid  on  more  resistant  silicates  like 
certain  natural  zeolites,  gives  a  much  more  granular-looking 
and  less  voluminous  silica.     The  action  of  boiling  hydro- 
chloric acid  on  still  more  resistant  silicates,  like  glauconite, 
leaves  a  silica  skeleton  with  the  exact  shape  and  dimensions 
of  the  original  mineral  grains.     In  like  manner  chrysotile 
on  boiling  with  dilute  acids  produces  silica  which  is  brilliantly 
white  and  fibrous  like  the  mineral ;    even  after  heating  to 


SILICA  7 

redness  it  still  retains  the  flexibility  of  silk ;  serpentine 
and  magnesian  silicates  generally,  yield  fibrous  and  not 
gelatinous  silica.  Even  when  prompt  and  abundant  pre- 
cipitation of  silica  occurs,  a  certain  quantity  always  remains 
in  the  state  of  colloidal  solution. 

Calcined  Silica  is  obtained  by  adding  hydrochloric 
acid  to  solution  of  sodium  silicate,  evaporating  to  dryness, 
taking  up  by  acidulated  water,  and  filtering,  the  insoluble 
deposit  being  well  washed,  then  dried  and  ignited. 

In  order  to  obtain  chemically  pure  silica,  silicon  fluoride 
is  decomposed  by  water. 

Chemical  Function  of  Silica. — Silica  gives  definite 
salts  with  bases.  Simple  silicates  of  protoxides  generally 
correspond  to  SiO2.MO  or  SiO2.2MO,  and  these  salts  may  be 
anhydrous  or  hydrated,  but  no  silicates  of  sesquioxides  are 
known  with  the  oxygen  in  base  and  acid  having  the  same 
ratios  (i  :  2  and  2:2). 

Action  of  Bases  on  Silica. — As  an  acid  body,  silica 
cotnbines  with  bases,  by  direct  action  or  by  double  decom- 
position, to  form  metallic  silicates.  Freshly  precipitated 
hydrated  silica  dissolves  almost  instantaneously  in  alkaline 
solutions.  Calcined  precipitated  silica  scarcely  dissolves 
in  the  cold,  but  rapidly  on  boiling.  Quartz  and  flint — even 
in  very  fine  powder — are  scarcely  attacked  by  boiling 
solutions. 

Flint  is  dissolved  easily  at  200°  C.  by  alkaline  solutions 
under  pressure,  this  being  one  of  the  industrial  processes  for 
making  sodium  silicate. 

Silica  combines  less  easily  with  alkaline  earthy  bases, 
because  of  the  insolubility  of  the  resulting  silicates.  This 
reaction  is  utilized  in  the  hydraulic  products  industry,  where 
certain  varieties  of  silica  combine  with  lime  and  cause 
hardening  of  the  mass.  Even  ground  quartz  combines  with 
lime  in  the  presence  of  water  under  pressure,  at  about  150° 
to  200°  C.  On  that  reaction  the  manufacture  of  sand-lime 
bricks  depends. 

Combination  of  every  variety  of  silica  is  effected  just  as 
readily  in  the  dry  way,  if  the  temperature  be  above  the 


8  SILICA   AND   THE  SILICATES 

fusing  point  of  the  silicates  formed.  This  combination  is 
produced  partially  at  lower  temperatures  with  very  finely 
divided  materials  mixed  very  uniformly.  The  reaction  is 
greatly  facilitated  in  this  case  by  the  presence  of  fusible 
compounds  serving  as  solvents,  as  in  the  case  of  lime  by  the 
presence  of  aluminous  and  ferruginous  compounds.  On 
this  property  is  based  the  manufacture  of  all  the  ordinary 
hydraulic  products,  viz.  hydraulic  limes  and  cement,  con- 
stituted essentially  by  silicates  of  lime,  and  burned  at 
temperatures  much  below  the  fusing  point  of  these  com- 
pounds, which  exceeds  2000°  C. 

Action  of  Alkaline  Salts. — Silica  is  the  weakest 
inorganic  acid,  and  its  heat  of  combination  being  lower  than 
that  of  all  the  other  acids,  its  substitution  for  any  one  of 
them  absorbs  heat.  This  reaction,  like  all  endothermic 
reactions,  will  become  the  more  complete  as  the  temperature 
is  raised.  Volatilization  of  the  displaced  acid  favours  the 
reaction. 

At  ordinary  temperatures  all  alkaline  and  alkaline  earthy 
silicates  are  immediately  decomposed  by  all  acids,  including 
even  the  weakest,  like  carbonic  acid.  On  boiling,  hydrated 
silica,  and  even  calcined  precipitated  silica,  dissolves  in 
concentrated  solutions  of  alkaline  carbonates,  the  carbon 
dioxide  being  continually  carried  away  by  water  vapour. 
The  reaction  is  favoured  by  concentration  of  the  solution  of 
carbonate  of  soda,  and  also  by  increase  of  the  total  mass  of 
carbonate  with  reference  to  the  total  weight  of  silica  to  be 
dissolved.  At  a  little  above  100°  C.  the  alkaline  nitrates 
begin  to  be  decomposed  by  silica.  At  a  red  heat  the  dis- 
placement of  carbonic  acid  by  any  variety  of  silica  (even 
quartz  sand)  is  instantaneous. 

The  alkaline  sulphates  begin  to  be  decomposed  above  a 
bright  red  heat,  but  even  at  1400°  C.  decomposition  is  not 
complete.  This  reaction  is  utilized  in  the  manufacture  of 
glass,  a  little  carbon  being  often  added  to  make  the  de- 
composition of  the  sulphate  more  complete,  and  to  avoid 
leaving  in  the  glass  too  much  of  that  salt,  which  in  crystal- 
lizing on  cooling  might  give  rise  to  white  opaque  spots. 


SILICA  9 

At  a  red  heat  silica  reacts  also  on  sodium  chloride  in 
the  presence  of  water  vapour,  thus  :  SiO2+2NaCl+H2O 
=SiO2.Na2O+2HCl.  This  reaction  is  applied  in  the  ceramic 
industry  for  salt-glazing  stoneware. 

Acids  in  general  do  not  act  on  silica,  excepting  hydro- 
fluoric and  phosphoric  acids. 

Action  of  Hydrofluoric  Acid  on  Silica. — Hydro- 
fluoric acid  acts  on  silica  from  ordinary  temperatures  up- 
wards, very  rapidly  on  precipitated  silica  (whether  calcined 
or  not),  and  more  slowly  on  crystallized  silica.  The  reaction 
gives  in  the  dry  way  silicon  fluoride  (gas) ,  and  in  the  wet  way 
hydrofluosilicic  acid  : 

Si02+4HF=SiF4+2H20 
SiO2+6HF=SiF4.2HF+2H2O 

The  first  reaction  is  only  possible  in  presence  of  excess 
of  gaseous  hydrofluoric  acid  to  oppose  the  inverse  reaction 
of  silicon  fluoride  on  water  (tending  to  reproduce  silica  and 
hydrofluoric  acid).  This  property  of  hydrofluoric  acid  is 
utilized  in  chemical  analysis  to  verify  the  purity  of  silica, 
which  should  volatilize  completely  with  excess  of  hydro- 
fluoric acid.  Hydrofluoric  acid  is  also  used  in  analysis  for 
silicates  in  which  it  is  only  proposed  to  determine  bases, 
much  time  being  saved  in  this  way. 

This  property  of  hydrofluoric  acid  is  also  used  for  etching 
glass  surfaces,  and  in  the  manufacture  of  cast  iron  for 
cleaning  cast  pieces,  which  often  retain  on  their  surface 
silica  and  silicates  from  the  moulding  sand. 

In  laboratories  the  limited  action  of  hydrofluoric  acid  on 
large  crystals  of  quartz  causes  corrosion  figures  to  appear. 

Action  of  Phosphoric  Acid  on  Silica. — Phosphoric 
acid  combines  with  silica  to  form  a  crystallized  compound  : 
P2O5.SiO2,  exactly  as  if  silica  were  a  base.  This  phosphate 
of  silica  occurs  in  four  distinct  allotropic  forms,  differing 
in  crystalline  form  and  chemical  properties.  Two  of  them, 
prepared  at  low  temperature,  are  more  or  less  rapidly 
attacked  by  water.  Two  others,  prepared  at  higher  tempe- 
rature, are  unaffected  by  water  and  even  by  hydrofluoric 


io  SILICA  AND    THE  SILICATES 

acid.  This  last  property  is  imparted  to  silicious  glasses  when 
phosphoric  acid  is  introduced. 

These  phosphates  of  silica  are  prepared  from  5  parts 
hydrated  silica  (obtained  from  decomposition  of  silicon 
fluoride  by  water)  mixed  with  100  parts  of  trihydrated 
phosphoric  acid,  P2O5.3H2O.  The  mixture  heated  to  260°  C. 
gives  at  first  a  transparent  mass,  all  the  silica  being  in  solution, 
but  this  solution  is  not  very  stable,  and  would  in  time  allow 
the  second  phosphate  of  silica  to  crystallize.  The  mass 
taken  up  again  by  alcohol  leaves  hexagonal  prisms,  macles, 
showing  energetic  double  refraction  and  changeable  by 
water.  Still  warming  slowly  the  clear  solution  prepared 
at  260°  C.,  it  soon  grows  muddy,  and  at  about  360°  C.  it 
deposits  numerous  crystalline  lamellae,  producing  a  felting 
in  the  whole  liquid.  These  lamellse  are  hexagonal,  have 
very  weak  double  refraction,  and  much  resemble  tridymite 
(a  low  specific  gravity  form  of  crystallized  silica).  But 
melted  silver  nitrate  gives  with  them  yellow  phosphate  of 
silver.  This  variety  is  decomposed  by  water  much  more 
slowly  than  the  preceding. 

The  same  solution  made  at  260°  C.,  then  heated  suddenly 
up  to  700°  or  800°  C.,  remains  at  first  clear,  but  if  maintained 
at  this  temperature  it  causes  gradual  deposition  of  octahedral 
crystals,  completely  isotropic,  of  phosphate  of  silica.  Water 
does  not  affect  this  variety.  The  density  is  3*41,  and  it 
melts  at  1400°  C.,  giving  a  vitreous  mass  which  does  not 
devitrify  on  cooling. 

A  solution  less  concentrated  in  silica  than  the  preceding, 
heated  between  900°  and  1000°  C.,  gives  no  octahedra,  but 
clinorhombic  prisms,  which  act  on  polarized  light,  but  are 
not  affected  by  water. 

Starting  from  metaphosphoric  acid,  P2O5.H2O,  and 
adding  pure  silica  (from  silicon  fluoride),  octahedral  phosphate 
can  be  prepared. 

Calcined  silica  and  crystallized  silica  do  not  seem  to  be 
attacked  in  the  same  conditions,  up  to  300°  C.,  but  at  a 
higher  temperature  all  the  varieties  of  silica  are  attacked  by 
phosphoric  acid,  (i) 


SILICA  ii 

Action  of  Reducing  Agents  on  Silica. — Carbon  has 
no  action  on  silica  below  1200°  C.,  but  in  the  electric 
furnace  reduction  begins  at  1450°  C.,  according  to  the 
equations  : 

Si02+2C=Si-f2CO 
SiO2+3C=SiC+2CO 

Silica  can  be  reduced  still  more  easily  by  heating  with 
certain  metals,  as  potassium,  magnesium,  or  aluminium, 
which  reduce  it  to  silicon.  Aluminium  is  sometimes  employed 
in  the  preparation  of  silicon.  By  throwing  into  a  heated 
crucible  (at  800°  C.)  a  mixture  of  aluminium  and  finely 
divided  silica,  a  violent  reaction  produces  a  mixture  of 
brown  silicon  and  alumina.  On  adding  potassium  silico- 
fluoride  to  the  mixture,  and  using  excess  of  aluminium,  a 
metallic  melted  bottom  is  obtained,  which  after  the  action  of 
hydrochloric  acid  leaves  crystals  of  silicon  (soluble  in  the 
melted  aluminium,  recrystallizing  on  cooling). 

The  presence  of  certain  metals — copper,  silver,  iron, 
platinum — favours  partial  reduction  of  silica  by  carbon 
through  absorption  of  the  silica.  On  heating  of  cast  iron 
(consisting  mainly  of  iron  and  carbon)  in  silica  crucibles, 
it  takes  up  from  the  walls  of  the  crucibles  several  per  cent, 
of  silicon.  The  same  reaction  takes  place  in  blast  furnaces 
at  the  expense  of  the  silica  of  the  slags  ;  it  is  more  complete 
the  higher  the  temperature,  and  the  less  basic  the  slags. 
Grey  cast  iron  includes  normally  i  to  2  per  cent,  of  silicon 
which  has  been  thus  reduced,  (i) 

Quartz  is  by  far  the  commonest  variety  of  silica,  and 
indeed  the  commonest  of  all  minerals.  It  occurs  in  various 
forms  and  under  different  conditions,  mostly  in  grains  or  in 
extensive  masses  of  a  white  or  grey  colour.  Sometimes 
quartz  occurs  of  a  red,  brown,  yellow,  blue,  green,  or  even 
black  colour.  The  transparent  colourless  crystals  known  as 
rock  crystal  (or  simply  crystal)  are  practically  pure  silica. 
Impure  quartz  may  be  almost  opaque.  Crystals  up  to 
20  inches  in  diameter  have  been  found  in  Brazil  and 
Madagascar,  whereas  grains  of  quartz  in  sand  are  often  less 


12  SILICA   AND    THE  SILICATES 

than  ^5  in.  in  diameter.  Naples  museum  has  an  aggregate 
of  quartz  crystals  weighing  about  a  ton. 

Quartz  possesses  in  a  very  marked  degree  circular 
polarization. 

It  also  possesses  the  special  property  of  piezo-electricity  ; 
it  can  transform  mechanical  energy  into  electrical  energy, 
and  conversely,  and  this  fact  has  been  applied  by  Curie  in 
the  construction  of  an  electrometer  of  high  precision. 

The  thermal  expansion  of  quartz  is  quite  abnormal,  as 
it  is  with  all  the  other  varieties  of  crystallized  silica. 

Crystalline  Symmetry  of  Quartz. — Most  quartz 
crystals  have  six  faces  (opposite  faces  being  parallel)  of  a 
hexagonal  prism,  with  a  hexagonal  pyramid  at  one  end  or 
at  each  end.  The  prism  is  sometimes  absent,  the  two 
pyramids  meeting  at  their  bases.  The  faces  are  often  very 
unequal  as  regards  size,  making  the  crystals  look  distorted, 
though  the  angles  between  corresponding  faces  remain  the 
same.  The  prism  faces  have  parallel  striations,  which  are 
horizontal  when  the  prism  is  placed  in  a  vertical  position, 
and  which  serve  to  distinguish  quartz  from  other  minerals 
of  similar  form.  Rather  rarely,  one  end  of  the  quartz 
crystal  is  terminated  by  a  plane  (instead  of  a  pyramid). 
In  some  specimens  the  terminal  hexagonal  pyramid  is  re- 
placed by  a  trihedral  one.  The  hexagonal  pyramid  is  really 
made  up  of  two  sets  of  faces,  the  alternate  faces  forming 
either  set  agreeing  with  one  another,  but  showing  differences 
from  those  of  the  second  set.  One  set  of  three  faces  has 
striae,  the  other  set  being  smooth  and  sometimes  slightly 
convex.  The  corrosion  figures  (produced  by  action  of  dilute 
hydrofluoric  acid)  always  show  a  marked  difference  between 
adjacent  faces  of  the  pyramid.  These  corrosion  figures 
show  lines  perpendicular  to  the  axis  of  the  crystal  (and 
parallel  to  the  base  of  the  triangular  face  of  the  pyramid)  on 
three  of  the  faces,  whereas  on  the  other  three  faces  the  lines 
are  inclined. 

The  symmetry  of  quartz  crystals  is,  in  fact,  only  ternary, 
and  not  hexagonal.  One  rhombohedron,  generally  better 
developed  than  the  other,  is  said  to  belong  to  the  direct 


SILICA  13 

form  (with  corrosion  striae  perpendicular  to  the  axis),  and 
the  other  to  the  inverse  form.  The  two  forms  of  rhombo- 
hedron  are  distinguished  as  right  and  left,  according  to  the 
direction  in  which  they  cause  rotation  of  the  plane  of 
polarized  light.  Occasionally  small  trapezoidal  faces  are 
found  replacing  the  corners  between  the  prism  and  pyramid 
faces. 

Inclusions. — liquid  inclusions  in  quartz  are  water  or 
liquid  CO2,  both  being  sometimes  included  in  a  bubble. 
Generally  a  little  nitrogen  is  present  with  the  CO2. 

Coloration. — Quartz  crystals  are  sometimes  coloured 
and  opaque.  At  other  times  different  minerals  are  visible 
in  the  interior  of  transparent  quartz  crystals. 

Fracture. — Quartz  crystals  show  no  cleavage  along 
definite  planes,  but  break  in  the  manner  of  glass  with  a 
bright  conchoidal  fracture,  and  small  fragments  have  sharp 
splintery  edges.  Sometimes  fractured  surfaces  show  a 
minute  rippled  marking  resembling  finger-prints. 

Specific  Gravity  is  never  less  than  2*65  for  quartz. 
The  mean  specific  gravity  of  quartz  at  o°  may  be  taken 
as  2*6507.  The  volume  of  the  inclusions  never  reaches 
o'i  per  cent,  of  that  of  the  crystal,  and  cannot  affect  more 
than  the  third  decimal. 

Expansion  on  Heating. — A  knowledge  of  the  anomalies 
is  very  important  for  the  application  of  silica  to  making 
refractory  products.  At  low  temperatures  (below  100°  C.) 
the  coefficient  of  expansion  has  been  found  to  be  nearly 
twice  as  great  perpendicular  to  the  axis  as  parallel  to  the 
axis.  Below  570°  C.  the  values  obtained  for  coefficients  of 
expansion  are  practically  the  same  as  those  for  the  low 
temperatures. 

At  570°  C.  there  is  a  sudden  expansion,  from  roo68  and 
i'on6  of  0*0034  an<i  0*00547  respectively.  Above  570°, 
after  this  sudden  and  reversible  modification,  the  expansion 
is  changed  into  a  slow  contraction.  This  behaviour  is  applied 
in  the  manufacture  of  silica  bricks  by  heating  the  quartz 
to  redness  before  submitting  it  to  crushing.  In  like  manner, 
the  ancients  are  said  to  have  used  fire  to  facilitate  the  cutting 


14  SILICA   AND   THE  SILICATES 

of  roads  in  the  granitic  rocks  of  the  Alps,  which  include  a 
large  proportion  of  quartz.  It  also  explains  difficulties 
met  with  in  earthenware  manufacture  in  trying  to  secure 
agreement  between  bodies  and  glazes.  The  body  always 
includes  quartz  (or  other  form  of  crystalline  silica)  in  some 
quantity,  and  owing  to  irregularity  of  expansion  it  is  im- 
possible to  find  a  glass  capable  of  agreeing  perfectly  with  the 
body.  In  porcelain  the  body  has  become  partly  vitreous, 
and  the  silica  has  assumed  the  amorphous  state.  In  hard 
porcelain  the  coefficient  of  expansion  is  nearly  constant  up 
to  1000°  C. 

Hardness. — Quartz  is  No.  7  on  the  hardness  scale  of 
Mohs,  and  is  harder  than  most  minerals,  including  all  the 
commoner  ones.  It  also  scratches  glass.  Certain  industrial 
applications  of  quartz  depend  on  its  hardness. 

Sandstone  formed  of  grains  of  quartzose  sand  serves 
to  make  millstones  to  use  for  grinding  and  for  cutting  hard 
bodies.  In  minor  industries  these  millstones  are  used  for 
sharpening  tools,  and  in  glass  manufacture  for  cutting  glass. 
Quartzose  sand  is  largely  used  as  abrasive  powder,  not  so 
hard  as  emery,  but  much  less  costly ;  it  is  largely  used  for 
polishing.  A  jet  of  sand  propelled  by  compressed  air  (sand 
blast)  has  received  numerous  industrial  applications. 

Compact  sandstones  and  quartzose  rocks  make  excellent 
building  materials.  Crushed  quartzites  are  very  good  for 
the  manufacture  of  concrete.  A  suitable  crusher  and  pulver- 
izer made  by  the  Patent  lightning  Crusher  Co.  is  shown  in 
Fig.  i,  p.  15. 

Specific  Heat  of  quartz  exhibits  peculiarities.  About 
600°,  during  transformation,  there  is  absorption  of  heat 
(5  calorie-grms.  per  grm.  of  substance).  At  higher  tempe- 
ratures the  specific  heat  tends  to  diminish,  contrary  to  all 
other  bodies  (except  certain  silicates),  (i) 

Circular  Polarization  of  light  by  quartz  is  exception- 
ally distinct. 

The  double  plate  of  quartz  is  formed  of  two  semicircular 
halves  (one  dextro-rotatory  and  the  other  laevo-rotatory)  cut 
perpendicular  to  the  prism  edges,  joined  along  a  vertical 


SILICA  15 

diameter.  The  thickness  of  the  plate  is  3*75  mm.,  enough 
to  turn  the  plane  of  polarization  of  yellow  rays  through  an 
angle  of  90°.  If  the  principal  section  of  the  analyser  be 
perpendicular  to  the  primitive  plane  of  polarization  the  two 
halves  of  the  extraordinary  image  will  show  the  transition 
tint,  and  will  thus  have  an  identical  coloration  as  if  there 
had  been  only  a  single  plate.  A  very  small  rotation  of  the 
analyser  will  cause  one  plate  to  change  to  red,  the  other  to 
blue,  and  so  accentuate  the  difference. 

In  order  to  determine  the  position  of  the  plane  of  polariza- 


FIG.  i. — Crusher  and  pulverizer. 

tion,  the  principal  section  of  the  analyser  has  only  to  be 
turned  until  the  tints  of  the  two  halves  of  the  extraordinary 
image  become  identical,  when  this  principal  section  will  be 
strictly  perpendicular  to  the  plane  of  polarization. 

Circular  polarization  by  other  substances  can  be  detected 
with  the  aid  of  the  same  apparatus.  The  effect  of  another 
substance  placed  in  front  of  the  double  plate  is  added  to 
that  of  one-half  of  the  plate  and  subtracted  from  the  other, 
and  the  result  being  the  same  as  for  a  difference  in  thickness 
of  the  two  plates,  identical  tints  cannot  be  obtained  for  the 


16  SILICA   AND   THE  SILICATES 

two  halves  of  the  image.  On  restoring  equality  by  means  of 
a  compensator,  the  rotatory  power  of  the  substance  intro- 
duced is  measured.  The  compensator  comprises  two  plates 
with  parallel  faces,  one  right,  the  other  left.  One  plate  has 
fixed  dimensions,  but  the  thickness  of  the  other  can  be 
varied — as  by  using  two  bevelled  plates,  of  which  one  can 
slide  over  the  other  while  keeping  the  two  external  faces 
always  parallel,  (i)  and  (2) 

Saccharimeters  are  instruments  for  measuring  the 
rotatory  power  of  certain  sugar  solutions,  etc.  By  measure- 
ment of  this  rotatory  power,  a  very  rapid  determination  can 
be  made  of  the  proportion  of  active  sugar  present  in  a 
solution. 

Double  Refraction. — When  a  ray  of  light  falls  obliquely 
on  a  transparent  plate  or  crystal  with  parallel  faces,  it  divides 
into  two  unequally  refracted  rays  on  entering,  and  emerges 
from  the  opposite  face  as  two  separate  rays,  parallel  to  the 
incident  ray  and  to  each  other.  The  distance  apart  of  these 
two  rays  increases  as  the  plate  becomes  thicker.  The  two 
emerging  rays  are  always  polarized  at  right  angles  to  one 
another,  and  on  this  peculiarity  depends  the  construction 
of  most  polarization  apparatus. 

Chalcedony  is  characterized  by  the  absence  of  trans- 
parency, and  usually  also  of  all  crystalline  form  visible  to 
the  naked  eye,  but  in  thin  sections  under  the  microscope  it 
shows  a  fibrous  structure  with  distinct  double  refraction. 
The  density  is  usually  between  2*55  and  2*58  (that  of  quartz 
being  2*65).  The  expansion  is  quite  similar  to  that  of 
quartz,  and  presents  the  same  anomaly  towards  600°.  The 
varieties  of  silica  included  in  chalcedony  are  generally 
impure ;  they  include  small  quantities  of  water,  and  seem 
to  be  transitional  between  hydrated  silica  and  quartzose 
silica.  Crystals  of  quartz  are,  in  fact,  often  found  on  one 
face  of  a  mass  of  chalcedony,  and  hydrated  silica  on  the 
other  face.  Chalcedony  often  occurs  as  concretionary 
masses,  formed  in  more  or  less  regular  beds,  and  sometimes 
distinguished  by  the  variety  of  colorations,  like  certain 
agates. 


SILICA  17 

Not  only  chalcedony,  but  all  terrestrial  rocks  include 
small  quantities  of  water  of  which  the  state  is  not  fully 
elucidated.  It  may  be  simply  lodged  in  voids  between 
the  fibres.  Chalcedony  is  not  absolutely  compact.  The 
transition  from  chalcedony  to  opal  seems  to  be  continuous, 
so  that  the  last  would  be  constituted  by  crystalline  elements 
too  fine  to  be  discernible  even  under  the  microscope.  Chal- 
cedony often  surrounds  foreign  materials  in  the  rock  in 
which  it  occurs,  notably  fossils.  Everything  indicates  that 
these  agglomerations  of  silica  are  formed  in  the  wet  way  and 
at  low  temperature,  which  is  not  generally  the  case  for  the 
other  varieties  of  silica.  Quartz  is  often  formed  above 
600°  C.,  as  its  corrosion  figures  prove,  and  other  varieties 
of  silica  with  low  density  at  temperatures  higher  still,  (i) 

Action  of  Heat  on  Chalcedony. — Chalcedony  begins 
to  lose  its  water  between  100°  and  200°  C.  Too  rapid 
heating  may  cause  rupture  with  explosion,  as  happens  with 
all  materials  having  very  fine  pores  impregnated  with  water, 
such  as  slate  and  porcelain  biscuit.  This  shivering  arises 
from  inability  of  the  water  vapour  to  escape  quickly  enough 
through  the  pores,  and  so  the  internal  pressure  increases 
very  rapidly.  This  dehydration  can  be  effected  without 
shivering,  not  only  for  chalcedonic  material  like  flint,  but 
for  slate  or  moist  unglazed  porcelain,  on  heating  these 
bodies  to  200°,  a  temperature  at  which  the  vapour  pressure 
is  not  great  as  compared  with  the  mechanical  strength  of  the 
solid  bodies  which  imprison  it.  The  water  vapour  is  dis- 
engaged slowly — sometimes  for  several  hours — and  after- 
wards the  temperature  can  be  raised  rapidly  without  any  risk. 

In  further  raising  the  temperature  of  chalcedony, 
ruptures  by  shivering  are  again  produced  towards  600°, 
from  exactly  the  same  cause  as  the  shivering  of  ordinary 
quartz  at  the  same  temperature.  After  this  transformation 
the  material  loses  its  translucency,  becomes  whitish,  and 
takes  a  permanent  increase  of  volume,  With  further  gradual 
rise  of  temperature  this  swelling  increases  regularly  until 
1000°  C.,  and  perhaps  higher.  The  permanent  expansion 
produced  at  1000°  C.  corresponds  to  a  linear  elongation 

c.  2 


i8  SILICA    AND  THE   SILICATES 

varying  according  to  the  sample  from  0*5  in  the  case  of  fossil 
woods  to  4  per  cent,  for  agates.  Successive  heatings  of  the 
same  specimens  do  not  cause  new  permanent  expansions 
so  long  as  the  temperature  of  the  first  heating  is  not  exceeded, 
but  the  sudden  and  reversible  change  at  about  600°  C.  always 
recurs. 

This  expansion,  and  the  loss  of  transparency  which 
accompanies  it,  seem  to  result  from  the  formation  of  in- 
numerable fissures.  The  calcined  samples  have,  in  fact, 
become  porous  and  capable  of  absorbing  liquids.  Their 
absolute  specific  gravities  have  not  diminished,  but  the 
apparent  volume  has  increased. 

When  heated  at  1400°  C.  in  the  porcelain  oven,  chal- 
cedony is  completely  transformed  into  a  variety  of  silica  of 
specific  gravity  about  2*3.  It  is  the  same  transformation 
that  quartz  suffers,  but  is  produced  more  rapidly  with 
chalcedony,  (i) 

The  more  important  varieties  of  quartz  may  be  grouped 
as  follows  (slightly  modified  from  L,.  J.  Spencer)  :  (2) 

A.  Crystallized  quartz  with  vitreous  lustre. 

B.  Quartz  with  enclosures  of  other  minerals. 

C.  Chalcedony  or  cryptocrystalline  quartz. 

1.  With  waxy  lustre. 

2.  Compact  varieties  with  dull  lustre. 

Not  included  in  these  are  a  few  varieties  which  are  rocks 
rather  than  minerals :  sand,  sandstone,  quartzite,  vein- 
quartz,  oilstone,  whetstone,  etc. 

Group  A  includes  rock-crystal,  amethyst,  citrine,  cairn- 
gorm and  smoky  quartz,  rose  quartz,  milk  quartz,  sapphire 
quartz,  and  prase.  The  name  crystal  or  rock-crystal — from 
the  Greek  krustallos,  "  clear  ice  " — originated  in  the  ancient 
belief  that  the  mineral,  which  seems  cold  to  the  touch  and 
commonly  occurs  in  rocks  among  Alpine  glaciers,  was  really 
congealed  water.  It  has  been  used  from  a  very  early  period 
for  making  ornaments,  carved  vases,  etc.,  sometimes  finely 
engraved,  and  lenses  or  globes  were  applied  for  kindling  fire 
and  other  heating  purposes.  Ladies  were  provided  with 
crystal  balls  to  cool  their  hands  in  hot  weather.  Rock 


SILICA  19 

crystal  is  still  used  as  a  cheap  gem-stone  or  ornamental 
stone,  and  for  making  spectacle  lenses,  though  in  some 
directions  largely  replaced  by  glass,  and  it  is  also  carved 
into  seals,  paper  weights,  and  spheres  for  crystal-gazing,  etc. 
L,ocally  it  often  receives  such  names  as  Bristol  diamond, 
Cornish  diamond,  Isle  of  Wight  diamond,  etc.  The  "  pebble" 
for  lenses  comes  chiefly  from  Brazil  and  Madagascar.  The 
finest  rock-crystal  in  Great  Britain  occurs  in  Cornwall  and 
about  Snowdon.  (See  below.) 

Amethyst  is  a  violet  or  purple  transparent  quartz  used 
as  a  gem-stone.  The  name  is  from  the  Greek  amethustos, 
"  not  drunken,"  because  of  the  ancient  belief  that  wearing  it 
prevented  intoxication.  Heating  has  the  effect  of  changing 
the  colour  of  amethyst  to  yellow,  and  much  of  the  yellow 
quartz  of  jewellery  is  said  to  be  produced  in  this  way  from 
amethyst.  The  origin  of  the  colour  in  amethyst  is  not 
known,  though  manganese  is  sometimes  stated  to  be  the 
cause.  Amethyst  is  composed  of  irregularly  alternating 
lamellae  of  right-handed  and  left-handed  quartz,  and  often 
gives  on  this  account  a  minute  rippled  fracture  suggesting 
finger  prints.  Citrine  includes  yellow  varieties  of  quartz, 
called  by  jewellers  "  occidental  topaz,"  "  Spanish  topaz," 
"  Scotch  topaz,"  etc.  These  yellow  varieties  are  some- 
times included  in  the  term  cairngorm  (from  the  name  of 
a  peak  of  the  Grampians  in  Banflshire),  which  also  includes 
the  brown  smoky  quartz.  The  latter  becomes  yellow  on 
heating,  but  the  yellowish-brown  colour  can  be  restored  by 
the  action  of  radium.  Smoky  quartz  sometimes  shows  the 
same  striated  structure  as  amethyst,  and  some  mineralogists 
include  with  the  latter  all  varieties  showing  such  structure, 
irrespective  of  their  colour.  The  brown  colour  of  smoky 
quartz  is  sometimes  deep  enough  to  make  the  stone  seem 
almost  black,  and  it  is  then  sometimes  termed  morion. 
Rose  quartz  has  a  pale  rose-red  colour,  but  is  never  found  in 
the  form  of  crystals  bounded  by  faces.  Milk  quartz  is  white, 
with  milky  opalescence.  Other  white  varieties  include  the 
potato-stone  from  Clifton  (Bristol),  which  is  hollow  and  lined 
with  crystals,  and  cotterite,  an  Irish  quartz  with  a  peculiar 


20  SILICA   AND   THE  SILICATES 

pearly  lustre ;  potato-stone,  however,  belongs  rather  to  the 
agate  group,  the  quartz  crystals  only  lining  the  interior. 
Sapphire  quartz  is  blue,  and  prase  of  a  leek  -  green 
colour. 

Group  B  includes  avanturine-quartz,  cat's-eye,  tiger-eye, 
hair-stone,  needlestone,  eisenkiesel,  etc.  Avanturine-quartz 
(or  aventurine-quartz)  is  spangled  throughout  with  minute 
yellow  scales  of  mica,  etc.  ;  it  is  found  in  Spain,  and  is  used 
for  ornaments,  like  other  members  of  this  group.  Cat's-eye 
quartz  exhibits  opalescence  due  to  enclosed  fibres  of  a  mineral 
resembling  asbestos.  Tiger-eye  has  a  golden  yellow  colour, 
and  consists  of  quartz  and  limonite  (hydrated  oxide  of  iron)  ; 
this  ferruginous  quartz  has  the  finely  fibrous  structure  of 
the  crocidolite  from  which  it  was  derived,  with  a  silky 
lustre  and  considerable  hardness  ;  similar  specimens  retain- 
ing the  indigo  blue  colour  of  the  crocidolite  are  known  as 
hawk's-eye.  Hair-stone,  needle-stone,  etc.,  enclose  needles 
of  rutile,  actinolite,  etc.  Eisenkiesel,  iron  flint,  or  ferru- 
ginous quartz,  encloses  iron  oxide  and  hydroxides,  which 
give  it  a  yellow  or  red  colour. 

Group  C  i  includes  agate,  onyx,  sardonyx,  sard,  carnelian, 
chrysoprase,  plasma,  bloodstone  or  heliotrope,  etc. 

Agate  is  a  natural  mixture  of  crystalline,  cryptocrystal- 
line,  and  colloidal  silica,  but  consists  mainly  of  chalcedony. 
The  name  is  derived  from  the  river  Achates  (now  the  Drillo) 
in  Sicily,  where  it  was  first  found.  In  most  agates  the 
silicious  material  occurs  in  regular  layers,  so  that  a  section 
gives  a  banded  appearance,  and  such  stones  are  sometimes 
termed  banded  agate,  riband  agate,  and  striped  agate. 
This  zoned  structure  generally  arises  from  silicious  matter 
being  deposited  from  solution  in  successive  layers  within 
cavities  formed  by  escaping  vapour  in  eruptive  rocks  or  old 
lavas.  Sometimes  such  deposition  has  taken  place  in 
fissures,  and  sometimes  in  cavities  of  stratified  rocks,  as  in 
the  potato-stones  mentioned  in  connection  with  Group  A. 
The  analysis  of  an  agate  gave  98*8  per  cent,  silica,  0*53  per 
cent,  ferric  oxide,  0*62  lime,  and  0*2  water.  Deposition 
of  silica  in  forms  very  similar  to  those  of  certain  agates, 


SILICA  21 

lias  been  effected  artificially.  The  cavities  in  rocks  may 
contain  various  minerals  resulting  from  decomposition  of 
the  constituents  of  the  rock.  The  first  deposit — forming 
the  "  skin  "  of  the  agate — is  generally  a  dark  green  earthy 
mineral,  delessite  (a  hydrated  silicate  of  aluminium,  iron, 
and  magnesium),  or  celadonite,  or  chlorophseite,  other 
hydrated  silicates  rich  in  iron,  probably  produced  by  the 
decomposition  of  the  augite  in  the  rock.  Subsequent 
deposits  often  fill  the  cavity  more  or  less  completely,  and  from 
the  commonly  almond-shaped  kernels  of  mineral  the  rock 
is  said  to  be  amygdaloidal.  This  shape  is  due  to  elonga- 
tion, by  the  flow  of  viscous  lava,  of  the  original  bubbles  of 
gas  or  steam.  When  the  mineral  kernel  is  hollow  it  forms 
a  geode  (or  cavity  lined  with  crystals),  but  when  nearly 
filled  with  alternating  layers  of  chalcedony,  jasper,  quartz, 
and  other  forms  of  silica  they  constitute  true  agates.  The 
green  skin  outside  an  agate  is  often  browned  by  the  formation 
of  limonite  (hydrated  oxide  of  iron)  from  the  green  mineral. 
The  largest  and  finest  agates  come  chiefly  from  Brazil  and 
Uruguay,  mostly  as  pebbles  from  the  beds  of  rivers.  The 
"  Scotch  pebbles  "  used  as  ornamental  stones,  found  chiefly 
in  Perthshire  and  Forfarshire,  similarly  occur  in  river  beds, 
but  also  in  pebble  drifts  on  the  land,  having  in  both  cases 
been  set  free  by  disintegration  of  the  rocks.  Agates  also 
occur  in  Northumberland,  in  Somerset  (Mendip  Hills  and 
near  Bristol),  and  about  L,ichfield  (Staffordshire).  They 
resist  the  action  of  air  and  water,  as  well  as  mechanical 
influences  such  as  abrasive  action,  better  than  most  minerals. 
In  ordinary  agates  the  successive  layers  are  variously 
curved  ;  in  fortification  agate  they  run  in  zigzag  lines  ;  when 
sections  show  concentric  circles  (due  to  stalactitic  or  stalag- 
mitic  growth)  the  stone  is  called  ring  agate  or  eye-agate,  and 
a  Mexican  agate  showing  only  a  single  eye  is  called  "  cy clops." 
Moss  agate  consists  of  chalcedony  enclosing  filaments,  etc. 
(mostly  green,  but  sometimes  red  or  brown),  suggestive  of 
vegetable  growths.  A  variety  of  agate  known  as  Mocha 
stone  consists  of  white  or  brown  chalcedony  with  black  or 
brown  dendritic  or  arborescent  markings  due  to  infiltration 


22  SILICA   AND   THE  SILICATES 

of  oxides  of  manganese  and  iron.  Imitation  Mocha  stone 
has  been  produced  artificially  at  Oberstein. 

Agate-working  has  long  been  practised  in  the  neighbour- 
hood of  Oberstein  (on  the  river  Nahe,  a  tributary  of  the 
Rhine).  Agates  were  formerly  obtained  in  the  district, 
but  they  are  mostly  imported  from  Brazil  and  Uruguay. 
Waldkirch,  like  Oberstein  in  the  Black  Forest,  has  also  been 
the  scene  of  some  agate-working. 

Most  commercial  agate  is  artificially  stained,  the  Brazilian 
agates  being  easily  coloured,  much  more  so  than  the  German 
stones.  For  a  dark  brown,  or  black  colour  the  stone  is 
kept  for  some  time  in  a  sugary  solution,  or  in  olive  oil,  at 
a  moderate  temperature.  The  agate  is  then  washed,  and 
digested  for  a  short  time  in  sulphuric  acid,  which  carbonizes 
the  sugar  or  oil  in  the  pores.  If  stained  too  dark,  the  colour 
may  be  "  drawn  "  or  lightened  by  the  action  of  nitric  acid. 
Some  layers  of  chalcedony  are  practically  impermeable  and 
remain  uncoloured,  so  that  usually  alternate  light  and  dark 
bands  are  obtained.  Agate  may  be  stained  red  or  brown 
to  simulate  carnelian  or  sardonyx,  by  means  of  ferric  oxide, 
either  formed  in  the  stone  by  simply  heating,  or  by  heating 
after  the  artificial  introduction  of  the  solution  of  a  salt  of 
iron  (such  as  ferrous  sulphate).  A  blue  colour  in  imitation 
of  lapis  lazuli  is  produced  by  first  using  ferrous  sulphate  and 
afterwards  a  solution  of  potassium  ferrocyanide  or  ferri- 
cyanide.  A  green  colour,  like  chrysoprase,  is  given  by  salts 
of  nickel  or  of  chromium,  and  a  yellow  tint  is  produced  by 
hydrochloric  acid. 

Among  the  purposes  for  which  agate  is  used  are  the  knife- 
edges  of  delicate  balances,  small  mortars  and  pestles,  bur- 
nishers and  writing  styles,  umbrella-handles,  paper-knives, 
trinket  boxes,  seals,  brooches,  beads,  and  other  small 
ornaments,  etc.  Most  of  them  have  been  cut  and  polished 
in  the  Oberstein  district,  from  South  American  agates. 

Onyx  is  a  variety  of  chalcedonic  silica  differing  from 
agate  only  in  the  straightness  and  parallelism  of  its  differently 
coloured  layers,  it  is  thus  suitable  for  engraving  as  cameos. 
The  alternate  bands  of  colour  are  usually  white  and  black 


SILICA  23 

or  white  and  red.  Most  of  the  onyx  with  strong  colour 
differences  are  now  coloured  artificially  in  the  same  way  as 
agates.  Sardonyx  is  much  used  for  seals  and  cameos,  and 
usually  consists  of  a  layer  each  of  sard  or  carnelian  and 
milk-white  chalcedony,  sometimes  several  such  alternating 
layers.  The  sardonyx  is  thus  simply  an  onyx  with  bands 
of  sard  or  carnelian,  though  in  the  latter  case  it  would  be 
more  correctly  called  a  "  carnelian  onyx."  Imitations  of 
sardonyx  have  been  made  by  cementing  together  two  or 
three  stones,  but  most  of  the  modern  sardonyx  is  cut  from 
South  American  agate,  coloured  artificially  as  mentioned  in 
connection  with  agate.  Sard  is  a  reddish-brown  or  yellowish- 
brown  chalcedony,  much  used  as  a  gem-stone  (for  cameos, 
etc.)  by  the  ancients.  Some  kinds  of  sard  greatly  resemble 
carnelian,  but  are  usually  rather  harder  and  tougher,  with 
duller  and  more  hornlike  fracture.  Both  are  chalcedonic 
quartz  coloured  with  iron  oxide.  The  colour  of  sard  ranges 
from  golden,  orange-red,  and  hyacinthine  to  a  dark  brown 
(almost  black).  Carnelian  or  cornelian  is  a  yellowish-red 
or  orange  coloured  chalcedony,  much  valued  for  engraving  ; 
its  fracture  has  a  peculiar  waxy  lustre,  distinct  from  that  of 
sard.  It  is  imitated  by  staining  agate.  Chrysoprase  is 
merely  a  pale  green  (apple  green)  chalcedony,  and  plasma 
is  a  darker  green  variety  (leek-green or  grass-green).  Helio- 
trope is  a  dark  green  chalcedony,  with  sometimes  bright  red 
spots,  streaks,  and  splashes,  when  it  is  called  bloodstone ;  the 
red  colour  is  due  to  hsematite,  which  is  itself  sometimes  called 
bloodstone.  Bloodstone  is  used  for  seals,  knife-handles,  and 
various  small  ornaments.  It  is  found  in  the  Isle  of  Rum. 

Certain  rare  varieties  called  lutecite,  quartzine,  and 
pseuda-chalcedonite,  present  slight  differences  of  structure 
from  chalcedony,  which  can  only  be  ascertained  by  the  aid 
of  the  microscope. 

Group  C  2  includes  jasper,  flint,  I/ydian  stone,  and 
buhrstone,  etc. 

Jasper  is  a  coloured  mixture  of  silica  and  clay,  with 
iron  oxide  and  hydroxide,  and  is  a  compact  opaque  mineral 
with  a  dull  earthy  fracture,  though  it  takes  a  good  polish. 


24  SILICA   AND  THE  SILICATES 

The  colours  are  chiefly  brown,  red,  yellow,  and  green  The 
impurities  present  with  the  silica  sometimes  amount  to 
20  per  cent.  Jasper  is  cut  and  polished  as  an  ornamental 
stone,  and  was  highly  prized  by  the  ancients.  Egyptian 
jasper,  which  occurs  as  nodules  and  pebbles  in  the  Nile 
Valley,  resembles  a  brown  flint  with  dark  zones  and  cloudings. 
Banded  jasper  is  a  striped  variety,  like  the  red  and  green 
"  riband  jasper  "  of  the  Ural  Mountains. 

Flint  is  a  dark  grey  or  dark  brown  variety  of  chalcedonic 
silica,  and  when  pure  shows  no  structure  to  the  unaided  eye. 
Though  individual  flints  look  dark  and  opaque,  thin  plates 
or  edges  of  splinters  are  pale  yellow  and  translucent.  It  is 
harder  than  steel,  and  its  specific  gravity  is  about  2*6.  It 
is  brittle,  and  when  hammered  it  readily  breaks  up  into 
powder  consisting  of  angular  grains.  The  fracture  is  con- 
choidal,  so  the  blows  of  a  hammer  detach  flakes  with  convex, 
slightly  undulating  surfaces.  At  the  point  of  impact  is 
produced  a  bulb  of  percussion — a  somewhat  elevated  conical 
mark — which  distinguishes  flints  thus  shaped  from  such  as 
have  been  split  by  the  action  of  frost  and  other  weather 
agencies.  Flint  often  contains  98  per  cent,  of  silica,  sometimes 
99  or  only  96,  and  also  contains  a  little  organic  material, 
to  which  the  dark  colour  seems  to  be  due,  as  it  becomes 
white  when  calcined,  but  with  duller  lustre.  In  micro- 
scope sections  flint  appears  very  finely  crystalline,  and 
colloidal  or  amorphous  silica  may  also  be  present ;  according 
to  Dr.  J.  W.  Mellor,  the  flints  used  in  pottery  manufacture 
have  about  ij  per  cent,  of  colloidal  silica.  Sponge  spicules 
and  microscopic  and  larger  shells,  often  occur  in  flints. 
Flint  occurs  chiefly  in  the  upper  chalk,  either  as  irregular 
nodules  or  as  tabular  masses.  The  white  crust  on  the  outside 
of  a  chalk  flint  contains  a  considerable  proportion  of  calcium 
carbonate,  the  other  constituents  being  practically  the  same 
as  in  the  black  core. 

Flint  is  often  used  for  building  purposes,  and  as  a  road 
material.  To  prehistoric  man  flint  served  many  of  the 
purposes  for  which  steel  is  now  used,  especially  for  making 
weapons  and  other  implements.  When  struck  with  steel  or 


SILICA  25 

with  iron  pyrites,  flint  gives  sparks  ;  hence  the  once  universal 
employment  of  flint  and  steel  with  tinder,  and  the  use  of 
gun  flints  in  the  locks  of  old  firearms.  The  shaping  of  gun 
flints  survived  till  recent  times  at  Brandon,  in  Suffolk,  about 
thirty  men  being  thus  employed  in  1870.  When  heated 
and  thrown  into  cold  water,  flint  can  be  easily  pulverized  ; 
the  white  powder  so  obtained  is  used  in  pottery  manu- 
facture and  to  some  extent  for  glass  making.  About 
30,000  tons  of  flint  per  annum  are  obtained  from  chalk  pits 
in  the  South  of  England,  and  a  similar  quantity  is  imported 
for  the  manufacture  of  pottery. 

Chert  or  hornstone  is  a  coarser  and  less  homogeneous 
substance  of  the  same  nature  and  approximately  the  same 
composition  as  flint,  but  it  usually  contains  more  oxide  of 
iron  and  lime.  Its  colour  is  grey,  black,  or  brown,  and  it 
commonly  occurs  in  limestone  (particularly  the  carboni- 
ferous limestone)  in  the  same  way  as  flint  occurs  in  chalk. 
Chert  is  extensively  worked  from  carboniferous  limestone 
quarries  in  Flintshire  and  Denbighshire,  Derbyshire,  and 
at  Reeth  in  Swaledale  (Yorkshire) .  It  is  used  in  connection 
with  potteries  for  paving  the  flint  mills. 

Lydian  Stone,  Lydite,  or  Flinty  Slate,  also  called 
basanite,  is  a  black  or  grey  cherty  material,  hard  and  close- 
grained,  which  has  been  used  as  a  touchstone  for  testing 
the  quality  of  gold.  The  metal  is  rubbed  on  a  polished 
surface  of  the  stone,  and  the  resulting  streak  is  compared 
with  standard  streaks  of  alloys  of  known  composition,  and 
also  tested  with  acid.  Another  use  of  Indian  stone  is  as 
a  hone-stone.  Most  Indian  stone  has  a  schistose  structure, 
and  may  be  regarded  as  a  hornstone  slate.  The  L,ydian 
stone  of  Devonshire  and  east  Cornwall  is  a  fine-grained, 
indurated,  carbonaceous  shale.  In  Sweden,  I^ydian  stone 
is  made  into  ornaments. 

Buhrstone  or  Burrstone  is  a  hard  tough  material 
consisting  of  chalcedonic  silica  with  a  cellular  texture, 
particularly  suitable  for  use  as  millstones  for  grinding  corn, 
paints,  etc.  It  is  white,  grey,  or  creamy  in  colour.  The 
best  stones  are  found  in  the  woods  of  Meudon,  Marly,  etc., 


26  SILICA   AND  THE  SILICATES 

about  Paris,  and  it  forms  the  best  building  stone  in  that 
neighbourhood.  The  cellular  spaces  represent  the  casts  of 
fossil  shells,  etc. 

Silicified  wood  is  a  comparatively  rare  form  of  chal- 
cedony, and  shows  the  microscopic  details  of  vegetable 
structure.  Tree  trunks  over  a  yard  in  diameter  have  been 
found. 

Sand  is  an  accumulation  of  grains  of  mineral  matter 
derived  from  the  disintegration  of  rocks.  When  rocks  or 
minerals  are  broken  up  by  either  natural  or  artificial  agencies, 
the  products  may  be  gravels,  sands,  and  muds  or  clays, 
according  to  the  size  of  the  constituent  particles.  When 
many  of  the  particles  are  as  large  as  peas  (or  larger)  the 
product  is  a  gravel,  and  when  the  material  forms  an  im- 
palpable powder  owing  to  the  extreme  fineness  of  the 
particles  (about  ycW  in-  across),  it  is  a  mud  or  clay  ; 
intermediate  grades  constitute  sands,  which  may  vary 
greatly  both  as  regards  size  of  grains  and  the  nature  of  the 
component  minerals.  Sands  which  have  been  sorted  out 
by  gentle  currents  of  water,  or  by  steadily  blowing  winds 
across  smooth  arid  lands,  may  be  uniform.  Natural  sands 
are  mostly  formed  by  the  action  of  the  air,  rain,  frost,  heat, 
plants,  and  other  agencies  in  breaking  up  the  surfaces  of 
rocks,  but  volcanic  sands  (consisting  of  fine  particles  of  lava) 
were  produced  by  explosions  of  steam  in  the  craters  of 
active  volcanoes.  A  few  sands  are  artificial,  like  the  crushed 
tailings  formed  in  the  extraction  of  metals  from  their  ores. 
Quartz  is  the  most  abundant  rock-forming  mineral,  and  its 
hardness,  want  of  cleavage,  and  chemical  properties  enable 
it  to  offer  a  strong  resistance  to  both  mechanical  and 
chemical  action  ;  it  survives  after  the  destruction  and  dis- 
persal of  the  accompanying  minerals,  and  accordingly  sands 
are  generally  silicious.  Much  of  the  earth's  surface  is 
covered  by  sand.  Fertile  soils  are  very  often  sandy,  though 
in  most  soils  sand  is  mixed  with  clay  or  stones,  and  such 
soils  should  be  described  rather  as  loams  than  sands.  Wind- 
blown sands  are  found  in  the  interior  of  deserts,  or  form 
sand-dunes  about  the  coast.  The  grains  of  desert  sand  are 


SILICA  27 

usually  much  more  rounded  than  those  of  water-borne  sand. 
The  abrasive  power  of  blown  sand  is  utilized  in  the  sand- 
blast, a  process  in  which  glass  or  stone  may  be  quickly 
engraved  or  cut  by  the  action  of  a  jet  of  sand  driven  with 
high  velocity  by  steam  or  compressed  air. 

Besides  quartz,  sands  often  contain  a  little  felspar,  but 
felspar,  though  very  common  in  rocks,  splits  up  easily  and 
then  decomposes  to  form  mica  and  kaolin ;  the  two  last- 
named  are  washed  away  by  water  and  are  deposited  as  muds 
or  clays.  Mica  is  often  mixed  with  the  quartz  and  felspar 
in  sands  ;  though  it  is  much  softer  than  felspar,  it  is  much 
more  resistant  to  decomposition,  but  may  become  very 
finely  divided.  Sands  also  contain  such  hard  and  resistant, 
common  rock-forming  minerals  as  garnet,  tourmaline, 
zircon,  rutile,  and  anatase.  Of  less  common  occurrence  in 
sands  are  topaz,  staurolite,  kyanite,  andalusite,  chlorite, 
iron  oxides,  biotite,  hornblende,  and  augite ;  in  the  coarser 
sands  are  often  small  particles  of  chert,  felsite,  and  other 
fine-grained  rocks.  Shore  sands  and  river  sands,  which 
have  not  been  carried  far  often  contain  soft  or  readily 
decomposable  minerals.  Thus  sands  about  the  Lizard  in 
Cornwall  are  rich  in  olivine,  augite,  enstatite,  tremolite,  and 
chromite.  Sands  near  volcanic  islands  may  contain  consider- 
able quantities  of  minerals  like  biotite,  hornblende,  augite,  and 
zeolites.  Marine  sands  nearly  always  contain  either  plant 
remains  or  bits  of  calcareous  shells.  Shell  sands  often  occur 
which  consist  almost  entirely  of  shell  fragments.  Coral 
sands  consist  of  coral  fragments,  mixed  with  broken  skeletons 
of  calcareous  algae,  sponge-spicules,  etc.  Green  sands  owe 
their  colour  to  dark-green  rounded  grains  of  glauconite. 
Many  sands  are  coloured  yellow  or  brown  by  hydrated 
ferric  oxide.  Gem-sands  (and  gravels)  occur  in  Ceylon, 
Burma,  Brazil,  South  Africa,  etc.,  and,  as  the  name  implies, 
contain  precious  stones  such  as  the  diamond,  ruby,  spinel, 
chrysoberyl,  and  tourmaline,  with  often  garnets  and  zircons. 
Gold  and  platinum,  etc.,  also  occur  in  sands,  and  so  does 
tinstone  as  "  stream  tin."  By  a  special  washing  process 
the  heavier  minerals  may  be  separated  from  sands.  (2)  and  (3) 


28  SILICA   AND   THE  SILICATES 

In  the  use  of  sand  for  nitration  of  water  on  a  large  scale, 
the  bed  of  filtering  material  consists  of  layers  of  graded  sand 
and  gravel.  At  the  bottom  is  placed  a  layer  of  gravel 
about  a  foot  thick,  the  size  of  the  pebbles,  and  fragments 
ranging  progressively  from  about  an  inch  at  the  bottom  to 
about  J  in.  at  the  top.  Above  the  gravel  is  a  layer  of 
fine  sand,  usually  several  feet  in  thickness.  The  main 
purpose  of  the  coarser  material  is  simply  to  support  the  fine 
sand,  which  forms  the  bulk  of  the  filtering  bed,  and  is  the 
real  filtering  agent.  The  sand  generally  used  has  a  grain 
size  of  0*2  to  0'4  mm.,  the  average  being  about  0*3  mm.  or 
a  little  more.  The  rate  of  filtration  depends  on  the  size  of 
grain  of  the  sand,  the  thickness  of  the  bed,  the  head  of  water, 
and  the  degree  of  turbidity  of  the  water.  The  suspended 
material  in  the  descending  water  is  arrested  by  the  top  layers 
of  sand,  rarely  penetrating  more  than  a  few  inches.  As 
the  filtering  bed  gets  clogged  with  accumulated  material,  the 
water  is  allowed  to  sink  through  the  sand,  the  top  layer  of 
which  is  then  scraped  off  to  expose  fresh  material.  At 
intervals  clean  sand  is  supplied  to  keep  the  bed  up  to  an 
effective  thickness.  Sometimes,  especially  when  filtration 
is  aided  by  vacuum  pressure,  the  surface  of  the  sand  is  pro- 
tected by  a  network  of  iron  or  wood,  so  that  the  deposit 
may  be  shovelled  away  without  greatly  disturbing  the  sand. 

Sands  are  put  to  many  uses.  Silicious  sands  practically 
free  from  iron  are  used  for  making  glass.  Sand  is  also  used 
for  making  mortar,  sand-lime  bricks,  and  artificial  stone. 
Another  important  use  is  for  polishing  and  scouring,  both 
for  domestic  and  manufacturing  purposes.  "  Bath  bricks  " 
are  made  from  the  sand  of  the  river  Parrett  (near  Bridgwater) . 
L,arge  quantities  of  sand  are  used  in  foundries  for  moulding 
purposes,  the  best  moulding  sands  containing  a  little  clay. 
Further  applications  of  sand  are  for  brick-making,  and  for 
filtering  water.  The  sand-blast  has  been  alluded  to.  A 
minor  application  is  the  use  of  desert  sand  (with  well- 
rounded  grains)  in  hour-glasses. 

Sandstone  is  a  consolidated  sand  rock,  consisting  of 
sand  grains  united  by  a  cementing  material.     When  the 


SILICA  29 

cement  is  weak,  the  friable  material  is  termed  sand-rock. 
The  size  of  the  particles  in  sandstones  varies  greatly,  and  may 
be  regular  or  irregular.  Coarse  sandstones,  called  grits, 
connect  sandstones  with  conglomerates  (composed  of  pebbles 
cemented  together),  and  fine-grained  sandstones  usually 
contain  some  mud  or  clay,  and  pass  very  gradually  into 
sandy  shales  and  clay  rocks.  Certain  very  impure  brown 
or  grey  sandstones  are  called  greywackes. 

Sandstones  contain  the  same  minerals  as  sands,  the 
commonest  of  them  being  quartz,  with  often  a  considerable 
amount  of  felspar,  and  usually  some  white  mica  as  well. 
Other  minerals  often  present  in  sandstones,  sometimes  in 
notable  quantity,  are  chlorite,  clayey  matter,  calcite,  and 
iron  oxides ;  whilst  garnet,  tourmaline,  zircon,  epidote, 
rutile,  and  anatase,  are  also  often  present,  but  with  rare 
exceptions  only  in  small  quantities.  Silicious  sandstones 
consist  almost  entirely  of  silica,  some  ganisters,  etc.,  Con- 
taining as  much  as  99  per  cent,  of  silica.  Felspathic  sand- 
stones or  arkoses,  which  contain  notable  amounts  of  felspar, 
are  less  durable  than  silicious  sandstones.  Micaceous 
sandstones  have  mica  flakes  arranged  along  planes  of  bedding. 
Argillaceous  or  clayey  sandstones  are  mixtures  of  sand  and 
clay.  Ferruginous  sandstones  are  brown  or  reddish,  due 
to  the  presence  of  ferric  oxide  (haematite)  or  hydroxide 
(limonite),  the  latter  brownish  yellow  and  the  former 
reddish,  which  form  thin  coatings  around  each  quartz  grain. 
Impure  sandstones  usually  consist  mainly  of  quartz  with  a 
considerable  admixture  of  other  minerals. 

The  consolidation  of  the  sand  may  result  either  from 
pressure  or  from  deposition  of  mineral  matter  between  the 
grains.  The  cementing  material  is  often  fine  chalcedonic 
silica,  sometimes  in  such  small  amount  that  even  with  a 
microscope  its  presence  is  not  easily  detected  ;  some  cherty 
sandstones  contain  more  chalcedony,  and  also  rounded 
dark  green  grains  of  glauconite,  and  such  green  sandstones 
belong  to  the  greensand  formation.  Silicious  sandstones 
are  very  hard  and  durable,  but  difficult  to  work.  Calcareous 
sandstone,  in  which  the  sand  grains  are  cemented  by  calcium 


30  SILICA   AND   THE  SILICATES 

carbonate  (calcite),  may  readily  be  worked  as  a  freestone, 
but  tends  to  become  disintegrated  by  weathering.  In 
Funtainebleau  sandstone,  Kentish  Rag,  Spilsby  sandstone, 
etc.,  the  calcite  is  in  large  crystalline  masses  which  on 
broken  surfaces  show  plane  cleavages  mottled  with  small 
rounded  sand  grains ;  the  Fontainebleau  sandstone  has 
rather  large  crystals  with  external  rhombohedral  faces. 
Argillaceous  sandstone,  which  contains  fine  clayey  material, 
has  been  consolidated  by  pressure,  but  remains  comparatively 
soft,  and  is  easily  broken  up  by  weathering,  etc.  In  ferru- 
ginous sandstone  the  iron  oxide  or  hydroxide  generally 
forms  only  a  thin  pellicle  about  each  grain,  but  in  some  cases 
they  are  more  abundant.  In  micaceous  sandstones  the 
mica  scales  often  facilitate  splitting  along  bedding  planes, 
as  in  Yorkshire  flagstone.  The  felspar  grains  in  f  el  spathic 
sandstones  are  either  crystalline  or  partly  decomposed  to 
form  kaolin,  as  in  much  of  the  English  millstone  grit,  which 
was  produced  by  disintegration  of  granitic  rocks.  Other 
cementing  materials,  found  only  in  certain  localities,  are 
dolomite,  barytes,  fluorite,  and  calcium  phosphate  (or 
phosphate  of  lime).  (2)  and  (3) 

The  colour  of  sandstone  is  mostly  due  to  impurity,  pure 
silicious  and  calcareous  sandstones  being  white,  creamy,  or 
pale  yellow  from  small  amounts  of  iron  oxides.  Black 
colour  arises  from  coal  or  manganese  dioxide,  red  from 
haematite,  yellow  from  limonite,  and  dark  green  from 
glauconite.  Grey  is  often  due  to  fragments  of  shale,  etc., 
but  in  many  sandstones  bluish  and  greyish  tints  are  referred 
to  the  presence  of  ferrous  carbonate,  finely-divided  iron 
pyrites,  or  even  iron  phosphate. 

Sandstones  are  much  used  as  building  stones,  and  are 
also  used  for  grindstones  and  millstones.  Very  pure  silicious 
sandstones  (such  as  ganisters)  are  used  for  lining  furnaces, 
hearths,  etc.  Sandstones,  being  porous,  absorb  water 
freely,  and  so  often  form  important  sources  of  water  supply 
(as  the  water  stones  of  the  Midland  Trias  in  England). 
Some  sandstones  are  so  impregnated  with  metallic  compounds 
that  they  form  useful  ores  :  among  others  occurring  in  this 


SILICA  31 

way   are  carbonates  of  copper,  sulphide  and  carbonate  of 
lead,  cobalt  and  manganese  ores,  metallic  copper,  etc. 

Quartzite  is  a  sandstone  formed  by  deposition  of 
crystalline  quartz  between  the  grains.  As  compared  with 
ordinary  sandstones,  quartzites  are  solid  quartz  rocks  free 
from  pores,  and  have  a  smooth  fracture,  the  break  passing 
through  the  sand  grains ;  in  ordinary  sandstones  the 
fracture  passes  through  the  cementing  material,  exposing 
rounded  faces  of  the  grains,  and  so  giving  the  broken  surface 
a  rough  or  granular  appearance.  Quartzites  are  too  hard 
and  splintery  to  be  at  all  largely  used  as  building  stones, 
but  are  much  used  for  roads,  though  in  spite  of  their  hardness 
they  are  readily  crushed  to  powder  if  not  well  embedded 
in  the  road  surface  ;  quartzite  of  Bmborough  (near  Bristol) 
and  of  Cherbourg  is  thus  used  for  roads. 

Oilstone  is  a  fine-grained  hone-stone  used  with  oil  for 
sharpening  edged  tools.  An  oilstone  from  Whittle  Hill  in 
Charnwood  Forest  (Leicestershire)  is  a  fine-grained,  silicious 
slaty  rock.  Welsh  oilstone  is  a  somewhat  similar  material 
from  near  I4yn  Idwal  in  North  Wales.  A  famous  oilstone 
conies  from  Asia  Minor.  In  Arkansas  (U.S.A.)  is  found  a 
bluish-white  fine-grained  oilstone  used  for  delicate  instru- 
ments. This  and  the  opaque  white  Washita  oilstone,  found 
by  the  Washita  River,  Arkansas,  are  called  novaculite.  Their 
material  is  99*5  per  cent,  chalcedonic  silica. 

Whetstone. — Very  fine  whetstones  for  sharpening 
edged  tools  are  made  from  novaculite  (named  from  nova- 
cula,  a  razor).  The  sharpening  quality  is  attributed  to  the 
presence  of  disseminated  crystalline,  silica,  or,  in  some 
varieties,  to  minute  garnet  or  rutile  crystals.  The  rock 
itself  has  to  be  cut  with  diamond  dust.  Some  of  the  pure 
white  novaculite  seems  to  be  a  silicious  deposit  from  hot 
springs,  other  varieties  being  fine-grained  altered  schists. 
"  Chocolate  whetstone  "  is  mica-schist  from  New  Hampshire, 
and  "  Hindostan  whetstone  "  is  from  Indiana.  "  German 
razor  hones  "  are  formed  of  a  fine-grained  argillaceous  rock 
found  in  slate  near  Ratisbon.  The  Scotch  "  Water-of-Ayr 
stone  "  or  "  snake  stone  "  is  used  for  rubbing  down  the 


32  SILICA   AND   THE  SILICATES 

surfaces  of  other  stones  and  of  copper-plates,  and  "  Tarn 
o'  Shanter  hones  "  are  used  as  ordinary  whetstones.  Coarse 
whetstones  for  scythes  have  long  been  worked  near  Black- 
down  in  Devonshire  under  the  name  of  "  Devonshire 
batts  "  ;  they  are  made  from  silicious  concretions  embedded 
in  sand.  Similar  stones  for  scythes  have  been  worked  at 
Benzlewood,  near  Stourton  (Wiltshire) . 

CRYSTALLIZED  SIUCA  OF  I^ow  SPECIFIC  GRAVITY. 

Several  varieties  of  crystallized  silica  with  specific 
gravity  between  2*25  and  2*35  are  known  They  occur  in 
very  small  crystals — perhaps  none  as  much  as  5  mm. — 
often  only  observable  in  compact  rocks  by  the  microscope. 
All  these  varieties  of  silica  are,  like  the  calcined  silica  of 
laboratories,  easily  soluble  in  concentrated  boiling  solutions  of 
sodium  carbonate. 

Tridymite  has  a  specific  gravity  2*28.  It  possesses  a 
rather  complex  crystalline  constitution,  but  extremely  con- 
stant so  that  it  is  easy  to  recognize.  It  was  described  by 
von  Rath  as  hexagonal,  but  Schuster  and  von  I/asaulx 
showed  that  it  was  not  uniaxial,  so  it  is  only  pseudo-hexa- 
gonal. Its  crystals  are  made  up  by  the  union  of  three 
orthorhombic  individuals — whence  the  name,  meaning  three 
twins.  The  network  of  tridymite  presents  great  analogies 
with  that  of  the  cube.  Groupings  are  observed  under  the 
constant  angle  of  70°  36',  which  is  very  near  the  70°  32'  of 
the  regular  octahedron.  The  hexagonal  appearance  is 
found  in  the  trachytes  of  Mont  Dore.  The  ordinary  appear- 
ance of  the  twin,  as  met  with  in  silicious  refractory  products, 
is  different. 

Optical  Properties  of  Tridymite. — The  mean  index 
of  refraction  for  the  D  ray  is  1*4775.  Double  refraction  is 
weaker  than  for  quartz,  the  colour  does  not  go  beyond 
grey. 

Merian  found  that  a  comparatively  small  rise  of  tempe- 
rature causes  in  tridymite  a  reversible  dimorphic  trans- 
formation, which  renders  it  positively  uniaxial,  and  it  is  then 


SILICA  33 

strictly  hexagonal.  Mallard  showed  that  this  trans- 
formation takes  place  at  130  ±  5.  This  transformation  of 
tridymite  explains  the  hexagonal  form  of  its  crystals. 
They  are  always  produced  at  temperatures  above  130° ; 
at  the  moment  of  their  formation  they  have  taken  the 
hexagonal  form,  and  on  cooling  have  kept  this  geometrical 
form,  in  spite  of  thainternal  transformation  of  their  network. 
The  transformation  at  130°  is  accompanied  by  an  abrupt 
change  in  the  linear  dimensions.  This  sudden  change  of 
dimensions  is  notably  less  than  that  of  quartz.  The  melting 
point  of  tridymite  is  1670°  C.,  as  recently  determined  by 
Ferguson  and  Merwin. 

Several  varieties  of  tridymite  occur,  one  of  which  is 
found  in  meteorites.  Tridymite  is  met  with  exclusively  in 
volcanic  rocks  and  their  recesses ;  or  in  the  refractory  pro- 
ducts of  works  (as  steel  works  or  glass  works),  and  then  only 
formed  at  high  temperatures.  It  is  one  of  the  essential 
elements  of  all  volcanic  lavas,  but  in  recent  lavas  only  occurs 
in  microscopic  crystals,  distinguishable  especially  by  their 
twinned  arrangement.  In  ancient  volcanic  rocks  (andesites 
and  trachytes)  it  is  sometimes  found  in  crystals  up  to  2  or 
3  mm.  in  size,  as  in  Auvergne.  L,avas  produced  artificially 
(by  M.  Levy  and  Fouque)  also  contained  tridymite  crystals, 
but  smaller. 

In  fragments  of  granite  rocks  or  sand  carried  away  by 
the  lava  stream,  and  heated  by  it,  the  quartz  is  partly  or 
wholly  transformed  into  tridymite. 

The  original  material  of  silica  bricks  is  quartz  with  a 
small  percentage  of  lime.  The  temperature  is  about  1600°  C., 
where  the  transformation  into  tridymite  takes  place. 

Cristobalite. — Von  Rath  discovered  in  the  trachyte  of 
San  Cristobal  (Mexico),  along  with  tridymite,  small  white 
octahedral  crystals  of  silica.  Mallard  showed  that  the 
angle  of  the  octahedron  is  70°  21',  very  near  that  of  the 
regular  octahedron.  The  specific  gravity  is  2*34,  and  its 
melting  point  is  1710°  ±10°  C.,  as  recently  determined  by 
Ferguson  and  Merwin. 

tridymite,  cristobalite  is  not  altered  by  heating  to 

3 


34  SILICA   AND   THE  SILICATES 

a  high  temperature.  Its  crystals  are  in  reality  quadratic, 
but  appear  pseudocubic  by  the  grouping  of  three  quadratic 
crystals  having  for  bases  the  three  faces  of  the  cube.  Each 
of  these  is  negative  uniaxial.  The  birefringence  (double 
refraction)  is  even  lower  than  that  of  tridymite. 

At  175°  C.  cristobalite  undergoes  reversible  dimorphic 
transformation,  and  above  this  temperature  its  birefrin- 
gence disappears  completely.  It  becomes  cubic,  which 
explains  the  octahedral  form  of  its  crystals,  produced 
certainly  at  a  high  temperature.  It  thus  differs  from 
tridymite  both  by  its  birefringence  and  by  its  point  of 
transformation  400°  higher. 

Cristobalite  occurs  also  at  Mayen  (Prussia),  as  pointed 
out  by  L,acroix. 

By  heating  in  a  platinum  crucible  for  five  hours  in  an 
autoclave  a  solution  of  colloidal  silica  with  a  little  boron 
fluoride,  ChroustschofT  obtained  colourless,  transparent,  and 
perfectly  isotropic  crystals  of  silica  (99*78  per  cent.  SiO2) 
with  specific  gravity  2*41  and  index  of  refraction  1*48. 
lyike  cristobalite,  the  crystals  were  isotropic  above  170°, 
and  feebly  birefringent  at  ordinary  temperatures,  and 
indeed  can  hardly  be  regarded  as  distinct  from  cristo- 
balite. 

By  heating  quartz — or  still  better  chalcedony  (or  flint) — 
to  a  very  high  temperature,  but  not  high  enough  to  cause 
formation  of  tridymite,  a  product  is  obtained  which  must 
be  considered  as  identical  with  cristobalite.  This  is 
characterized  by  abnormal  expansion,  with  sudden  increase 
of  linear  dimensions  at  about  210°  C.,  amounting  to 
i  per  cent.  Quartz  in  crystals  is  not  appreciably  changed 
after  24  hours  in  a  porcelain  oven  at  1400°  C.,  whereas  the 
same  quartz  finely  ground  (to  pass  a  200-mesh  sieve)  is 
completely  transformed  by  the  same  heating.  The  presence 
of  lime  in  contact  with  the  silica  promotes  this  change, 
whilst  alkalies  give  amorphous  silica  instead. 

The  production  of  this  cristobalite  explains  many  irre- 
gularities and  accidents  of  manufacture,  as  the  cracking  of 
fine  faience  (earthenware),  and  also  supplies  a  reason  for 


SILICA  35 

certain  manufacturing  processes  which  were  developed 
empirically.  These  will  come  in  for  further  consideration 
later. 

Small  crystals  of  quartz  were  first  produced  artificially 
in  1851  by  De  Senarmont  on  heating  gelatinous  silica  to 
350°  C.  in  a  sealed  glass  tube,  and  in  1857  (along  with 
wollastonite  crystals)  by  Daubree  by  simply  heating  pure 
water  in  ordinary  glass  tubes  (enclosed  in  iron  tubes)  at 
320°  C.  for  several  months.  In  1879  Friedel  obtained  them 
more  readily  at  a  higher  temperature,  and  using  a  little 
potash  solution  instead  of  acid  with  the  precipitated  silica. 
In  1883  ChroustschofI  obtained  small  quartz  crystals  from 
gelatinous  silica  with  a  little  boron  fluoride  by  heating  to 
about  300°  C.,  whilst  at  lower  or  higher  temperatures  other 
varieties  of  silica  were  obtained. 

In  1878  Hautefeuille  first  produced  quartz  crystals  in 
the  dry  way  by  dissolving  silica  in  certain  melted  salts. 
When  the  temperature  of  the  fused  material  exceeded  a 
certain  limit,  only  crystals  of  low  specific  gravity  forms  of 
silica  were  obtained.  In  1906,  Day  and  Shepherd  pro- 
duced distinct  crystals  of  quartz  by  heating  silica  to  700°  C. 
and  lower  with  80  parts  potassium  chloride  and  20  parts 
lithium  chloride. 

Tridymite  has  never  been  produced  artificially  in  the 
wet  way,  but  has  easily  been  obtained  in  very  imperfect 
crystals  by  the  prolonged  heating  of  silicious  glasses,  as 
when  Fouque  and  M.  L,evy  reproduced  volcanic  lavas. 

In  1869,  G.  Rose  first  announced  the  production  of 
tridymite,  but,  according  to  I^e  Chatelier,  that  may  have 
been  a  phosphate  of  silica,  which  closely  resembles  tridy- 
mite, but  was  not  identified  until  later  by  Haute- 
feuille. (i) 

The  production  of  tridymite  was  also  reported  by  Haute- 
feuille and  Parmentier,  and  by  Day  and  Shepherd  ;  here, 
too,  I<e  Chatelier  considers  that  more  recent  researches  of 
L,acroix  indicate  that  cristobalite  rather  than  tridymite 
was  obtained.  When  fluxes  are  used  a  low  specific  gravity 
form  of  crystalline  silica  is  certainly  obtained  between 


36  SILICA   AND   THE  SILICATES 

800°  and  1000°  C.,  but  whether   tridymite  or   cristobalite 
seems  doubtful. 

Herschkowitsch  showed  that  vitreous  silica  is  devitrified 
towards  1300°  C.,  and  the  specific  gravity  of  2*33  for  the 
resulting  confused  crystalline  material  seems  to  indicate 
the  formation  of  cristobalite. 

Regions  of  Stability. — Quartz  is  stable  at  low  tem- 
peratures, and  at  300°  and  700°  C.  (at  which  it  is  produced 
in  the  wet  and  dry  ways  respectively).  It  is  possible  that 
chalcedony  may  be  the  stable  form  under  some  conditions 
rather  than  quartz.  When  heated  rapidly,  quartz  reaches 
its  softening  point  without  being  transformed.  When 
maintained  near  1500°  C.  for  a  long  time,  quartz  becomes 
completely  transformed  into  tridymite,  and  at  much  lower 
temperatures  if  finely  powdered,  as  previously  mentioned. 
The  upper  limit  of  stability  of  quartz  is,  in  fact,  only  about 
800°  C.,  at  which  temperature  transformation  takes  place, 
whilst  the  product  changes  back  into  quartz  at  750°  C.  at 
atmospheric  pressure.  Increased  pressure  raises  the  tem- 
perature of  transformation. 

Tridymite  is  the  stable  variety  of  silica  above  1500°  C. 
(and  near  the  fusing  point).  The  lower  limit  of  temperature 
for  stability  of  tridymite  is  not  known.  It  was  stated  to 
coincide  with  the  upper  limit  of  quartz,  but  I,acroix  showed 
that  Day  and  Shepherd  had  obtained  cristobalite  and  not 
tridymite. 

I^e  Chatelier's  experiments  showed  that  the  low  specific 
gravity  silica  obtained  in  laboratory  experiments,  by 
heating  between  1200°  and  1400°  C.  bricks  made  of  fine  sand, 
possessed  neither  the  same  expansion  properties  nor  the  same 
transformation  point  as  tridymite,  but  rather  agrees  with 
cristobalite.  There  is  apparently  an  interval  of  temperature 
between  the  region  of  stability  of  quartz  and  that  of  tridymite. 

L,e  Chatelier  observes  that  if  cristobalite  has  its  own  zone 
of  stability  it  should  occur  more  frequently  in  nature,  at 
least  in  volcanic  or  metamorphic  districts.  Its  very  weak 
double  refraction  renders  its  observation  rather  difficult 
when  isolated  crystals  do  not  occur  in  the  geodes. 


SILICA  37 

Notwithstanding  doubtful  points,  these  reciprocal  trans- 
formations of  silica  play  an  important  part  in  geological 
phenomena,  and  in  certain  operations  in  ceramics. 

Hydrated  Silica. — The  proportion  of  water  usually 
lies  between  3  and  12  per  cent,  and  varies — even  in  the  same 
sample — with  external  temperature  conditions  and  hygro- 
scopic state.  These  hydrates  of  variable  composition  are 
not  crystallized,  but  occur  in  gelatinous  masses  or  in  clots 
without  showing  any  geometrical,  form  or  any  action  on 
polarized  light.  (Certain  zeolites,  hydrated  silicates  in  very 
fine  crystals,  also  possess  a  content  of  water  varying  with 
atmospheric  conditions.) 

Hydrated  silica  gives  so-called  colloidal  solutions,  which 
easily  freeze  into  jellies.  Artificial  hydrated  silica  has  been 
shown  to  contain  a  variable  quantity  of  water,  like  the 
natural  material. 

Coagulation  of  Hydrated  Silica. — The  mechanism 
is  as  follows  : — At  a  given  moment  the  homogeneous  material 
is  divided  into  pure  water  and  silica  retaining  less  water  than 
before.  One  of  the  liquids  is  resolved  into  small  independent 
drops,  and  the  other  forms  a  network  uniting  the  first ;  the 
second  becomes  more  or  less  solid  and  makes  the  mass 
pasty.  This  second  portion  is  often  separated  into  isolated 
clots  which  remain  suspended  in  a  continuous  mass  of  water. 
A  jelly  or  gel  consists  of  a  continuous  mass  of  the  semi-solid 
material  with  isolated  little  drops  of  pure  water  in  the  middle 
of  it. 

The  gel  contracts  progressively,  causing  the  water  to 
ooze.  The  water,  beginning  with  SiO2.30oH2O,  is,  by 
dripping  a  long  time  on  a  glass  plate  sheltered  from  evapo- 
ration, reduced  to  less  than  SiOa.iooI^O.  By  compression 
between  porous  bricks,  under  a  pressure  of  0*5  kg.  per  sq.  cm., 
more  water  is  expelled,  reducing  to  about  SiO2.2oH2O.  The 
tension  of  water  vapour  exercised  by  the  material  remains 
invariable  all  this  time,  and  is  equal  to  that  of  pure  water. 

Desiccation  goes  on  slowly  when  drained  gels  are  exposed 
to  the  atmosphere  at  definite  vapour  tension  below  that  of 
pure  water. 


38  SILICA   AND  THE  SILICATES 

According  to  I,e  Chatelier,  the  only  plausible  hypothesis 
is  that  silica  does  not  form  hydrates,  but  always  exists  in 
the  anhydrous  state,  even  in  "  hydrated  "  silica.  Other 
acids,  also,  as  chromic  acid,  do  not  form  hydrates  in  presence 
of  water. 

The  existence  in  gelatinous  silica  of  a  material  as  hard 
as  anhydrous  silica  should  make  it  applicable  for  polishing 
metals,  etc.,  a  deduction  which  has  been  confirmed 
experimentally. 

Natural  hydrated  silica  includes  the  varieties  of  opal. 

Opal  is  a  natural  colloidal  silica,  commonly  occurring 
in  nodular  and  stalactitic  forms  in  cracks  and  cavities  of 
volcanic  rocks.  The  name  comes  from  lyatin  opalus,  "  a 
precious  stone."  Its  hardness  is  about  6  (5*5  to  6-5),  and  its 
specific  gravity  ranges  from  1*9  to  2*3.  The  proportion 
of  water  usually  varies  from  about  2*5  per  cent,  to  n  or 
12  per  cent.  Unlike  quartz,  opal  is  almost  entirely  soluble 
in  hot  solution  of  caustic  alkali.  Opal  is  normally  isotropic, 
like  other  non-crystalline  minerals,  but  many  varieties, 
especially  of  noble  opal,  are  strongly  birefringent,  probably 
owing  to  strain  set  up  by  unequal  contraction  in  different 
directions  during  solidification  of  the  originally  gelatinous 
material.  The  index  of  refraction  for  the  middle  part  of 
the  spectrum  is  1*437  to  1*455. 

When  heated,  opal  loses  water,  cracks,  becomes  pearly, 
and  often  preserves  its  original  coloration.  Certain  opals 
heated  in  concentrated  sulphuric  acid  blacken  like  jet, 
keeping  still  coloured  reflections ;  they  include  a  little 
carbonaceous  matter.  A  Hungarian  noble  opal  with  less 
than  i  per  cent,  of  impurities  contained  in  the  natural  state 
ii  per  cent,  of  water,  and  after  desiccation  at  16°  and  100° 
the  water  was  9  and  4*7  per  cent,  respectively ;  these  two 
last  correspond  to  3SiO2.H2O  and  6SiO2.H2O  respectively, 
and  are  proportions  found  rather  often  in  other  varieties  of 
hydrated  silica. 

Hyalite  (from  Greek  hualos,  "glass,")  glassy  opal,  or 
water-opal,  sometimes  called  "  Miiller's  glass,"  is  a  colour- 
less transparent  variety  of  opal  with  little  water.  It  is 


SILICA  39 

found  chiefly  at  a  few  places  in  Bohemia,  Mexico,  and 
Colorado,  U.S.A. 

Semi- opal  includes  the  dull,  opaque  varieties,  which 
are  generally  more  or  less  impure. 

Common  Opal  is  a  name  applied  to  any  opal  which  has 
not  a  sufficient  display  of  colour  for  ornamental  purposes, 
including  milk-opal,  wax-opal  or  resin-opal,  agate-opal, 
prase-opal,  jasper-opal,  rose-opal,  etc.,  the  appearance  of 
which  is  suggestive  of  the  stone  or  other  substance  forming 
part  of  the  name.  Common  opal  is  often  found  in  the 
vesicular  lavas  of  County  Antrim  and  the  West  of  Scotland, 
etc. 

Fire  Opal,  sometimes  called  "  girasol,"  is  found  chiefly 
in  Mexico,  and  has  a  brilliant  orange  or  hyacinthine-red 
colour,  which  gives  it  some  value  in  jewellery. 

Noble  Opal  or  Precious  Opal  is  unique  in  its  brilliant 
flashes  of  iridescent  colours  by  reflected  light.  This  play 
of  colour  has  been  ascribed  to  the  presence,  within  the 
substance  of  the  opal,  of  numerous  microscopic  fissures  or 
pores  or  delicate  striae,  but  H.  Behrens  attributes  the  effect 
to  very  thin  lamellae  of  opaline  silica  (or  foreign  matter) 
having  a  different  refractive  index  from  that  of  the  matrix. 
In  the  variety  called  by  jewellers  "  harlequin  opal,"  the 
colours  flash  from  small  angular  surfaces,  forming  a  sort  of 
brilliant  mosaic.  In  other  varieties  the  colours  are  dis- 
tributed in  broad  bands  or  comparatively  large  irregular 
patches.  The  finest  opals  occur  in  Hungary,  along  with 
common  opal,  in  an  altered  andesite.  Slabs  of  the  matrix, 
or  "  mother-of-opal,"  enclosing  brilliant  particles  of  opal, 
are  polished  as  ornamental  stones.  Precious  opal  is  also 
found  in  Honduras,  Mexico,  Queensland,  and  New  South 
Wales.  Opals  are  usually  cut  with  convex  surface,  but 
when  too  thin  for  this,  as  is  often  the  case  in  Queensland,  the 
opal  is  used  for  inlaid  work,  or  is  carved  into  cameos  with 
the  dark-brown  ferruginous  matrix  forming  an  effective 
background.  The  "  root-of-opal "  consists  of  opal  dis- 
seminated through  the  matrix.  The  matrix  penetrated  by 
veins  and  spots  of  opal,  and  perhaps  heightened  in  colour 


40  SILICA   AND  THE  SILICATES 

by  artificial  treatment,  has  been  termed  "  black  opal,"  but 
true  black  opal,  enclosing  patches  of  manganese  oxide  and 
showing  a  brilliant  play  of  colours,  occurs  in  New  South 
Wales. 

Menilite  or  Liver  Opal  is  a  grey  or  brown  opaque  opal 
occurring  as  nodules  of  a  special  lenticular  form  about 
Paris,  especially  at  Menilmontant. 

Hydrophane  is  a  porous  opal  which  is  opaque  and  dead 
white  when  dry,  but  which  becomes  more  or  less  transparent 
when  immersed  in  water,  and  sometimes  gives  a  play  of 
colours.  A  fine  variety  from  Colorado  has  been  sold  in 
America  as  "  magic  stone."  Tabaschir  is  a  similar  silicious 
and  concretionary  matter  deposited  in  the  nodes  of  bamboos. 

Cacholong  is  another  kind  of  porous  opal,  with  somewhat 
pearly  lustre.  It  occurs  at  the  Giant's  Causeway  (Antrim), 
etc.  (4). 

Wood  Opal  or  Lithoxylon  is  simply  silicified  wood  or 
resinite  which  has  been  fossilized  with  opaline  silica.  It 
appears  similar  to  that  formed  from  chalcedony,  and  only 
differs  in  the  larger  water  content.  In  cut  and  polished 
specimens  the  woody  structure  produces  a  pleasing  effect,  and 
the  material  is  therefore  used  in  slabs  as  an  ornamental  stone. 

Besides  the  compact  forms  of  opal,  loose  and  friable 
forms  of  opaline  silica  occur  as  silicious  sinter  or  geyserite, 
kieselguhr,  etc. 

Geyserite  or  Silicious  Sinter  is  opaline  silica  deposited 
by  spontaneous  evaporation  from  hot  springs  (geysers)  in 
Iceland,  New  Zealand,  and  the  Yellowstone  National  Park 
(Wyoming,  U.S.A.).  It  is  fibrous  and  mammillated,  opaque 
in  mass,  but  composed  of  small  transparent  globules  im- 
prisoned in  an  apparently  gelatinous  paste,  and  includes  a 
high  proportion  of  water. 

Fiorite  or  Pearl  Sinter  is  opaline  silica  with  a  rather 
pearly  lustre,  from  the  hot  springs  of  Santa  Fiora  (Tuscany). 

Pealite  is  a  variety  with  an  exceptionally  small  pro- 
portion of  water,  from  the  Yellowstone  Park. 

When  very  loose  in  texture,  silicious  sinter  is  often 
called  silicious  tuff. 


SILICA  41 

Kieselguhr,  Diatomite,  Diatomaceous  Earth,  In- 
fusorial Earth,  Tripoli  (or  Tripolite),  Randanite,  is 
made  up  of  the  minute  silicious  shelly  remains  of  diatoms 
and  radiolarians  (but  especially  the  former),  which  accumu- 
lated at  the  bottoms  of  ancient  lakes,  often  forming  ex- 
tensive deposits,  usually  of  a  white  or  greyish  colour.  The 
purest  varieties  consist  of  88  per  cent,  silica,  10  per  cent, 
water,  and  2  per  cent,  basic  oxides.  Salvetat  found  in 
randanite  from  Auvergne  that  the  water  content  varied 
with  desiccation  (as  with  opal),  the  amount  after  desiccation 
at  1 6°  and  100°  being  9*1  per  cent,  (corresponding  to 
3SiO2.H2O)  and  47  per  cent,  (corresponding  to  6SiO2.H2O) 
respectively. 

Tripoli,  a  more  compact  and  laminated  variety  from 
Tripoli  and  Bohemia,  is  used  as  polishing  material,  and 
kieselguhr  is  used  as  absorbing  material  for  nitro-glycerine 
in  the  manufacture  of  dynamite. 

Elongated  forms  of  diatoms  like  naviculse  make  good 
insulating  material,  having  the  greatest  porosity. 

Kieselguhr  is  mostly  obtained  from  Naterleuss,  between 
Hamburg  and  Hanover,  where  the  deposit  extends  about 
150  ft.  downwards  from  the  surface,  and  three  portions  are 
distinguishable.  The  top  stratum  consists  of  white  kiesel- 
guhr, and  contains  some  sand,  but  very  little  organic  matter  ; 
the  washed  product  is  very  pure  and  porous.  The  next 
stratum  produces  grey  kieselguhr,  and  contains  very  little 
sand,  but  sufficient  organic  matter  for  calcining  the  material ; 
when  calcined  it  yields  the  best  quality  of  kieselguhr.  The 
lowest  stratum — and  much  the  largest,  being  50  to  100  ft. 
thick — produces  green  kieselguhr,  containing  up  to  30  per 
cent,  of  organic  matter ;  the  dry  material  when  heated 
glows  like  peat.  It  is  calcined  in  small  furnaces,  which, 
when  filled,  are  lighted  at  the  bottom,  and  no  additional 
fuel  is  needed ;  the  furnaces  are  continually  fed  with  the 
raw  material  at  the  top,  and  the  calcined  product,  which  has 
a  reddish  colour  (due  to  ferric  oxide)  is  taken  out  from  the 
grates  below. 

In  Scotland,  deposits  of  diatomite  occur  with  peat  in 


42  SILICA   AND   THE  SILICATES 

several  localities,  and  in  the  Isle  of  Skye  beds  up  to  40  ft. 
thick  have  been  worked.  In  Ireland,  kieselguhr  has  been 
found  in  County  Antrim.  In  the  United  States  enormous 
deposits  occur  at  several  places,  sometimes  reaching  a 
thickness  of  1000  ft.  (2) 

Kieselguhr  varies  greatly  in  composition,  the  general 
range  being  70  to  80  per  cent,  silica  (largely  in  the  soluble 
opaline  form),  10  to  20  per  cent,  water  and  organic  matter, 
with  small  amounts  of  alumina,  ferric  oxide,  lime,  etc.  Its 
high  non-conducting  power — due  to  its  great  porosity — fits 
it  for  coating  steam  boilers  and  pipes.  It  is  also  used  by 
safe  makers,  cooking  stove  and  kitchener  manufacturers, 
for  filling  up  bulkheads  in  steamships,  for  making  fireproof 
rooms,  etc.  It  has  been  made  into  fireproof  bricks  of  low 
specific  gravity  for  the  setting  of  steam  boilers,  the  lining 
of  blast  furnaces  and  the  associated  hot  air  pipes,  and  for 
the  backs  of  fireplaces.  It  has  further  been  suggested  as 
an  absorbent  for  bromine  for  disinfecting  purposes,  and  for 
making  "  dry  sulphuric  acid "  by  saturating  calcined 
kieselguhr  with  sulphuric  acid,  the  product,  which  contains 
75  per  cent,  of  its  weight  of  sulphuric  acid,  being  said  to  be 
transportable  by  land  or  sea  in  iron  vessels  without  damage. 

Kieselguhr  is  used  by  soap  makers,  by  manufacturers 
of  ultramarine,  and  for  making  imitation  meerschaum. 

Amorphous  Silica. — Calcined  silica  obtained  by  calcina- 
tion of  any  variety  of  hydrated  silica,  whether  from  action 
of  acid  on  sodium  silicate  or  decomposition  of  silicon  fluoride 
by  water,  is  usually  described  as  amorphous  silica.  The 
impalpable  powder  of  which  this  silica  consists  evades  all 
means  of  optical  investigation,  so  it  is  not  known  whether 
it  is  crystalline  or  not.  The  expression  "  amorphous  silica  " 
is  therefore  inappropriate  in  this  case.  The  region  of  stability 
of  the  amorphous  state  always  corresponds  to  higher  tem- 
peratures than  those  of  the  crystallized  state,  so  it  would 
seem  logical  to  suppose  this  material  were  crystallized, 
because  of  the  low  temperature  at  which  it  is  obtained. 

Fused  Quartz  was  the  first  really  amorphous  silica 
obtained — by  Gaudin,  about  1839.  The  melted  quartz  on 


SILICA  43 

cooling  did  not  crystallize  like  alumina,  but  lost  its  double 
refraction  and  formed  a  true  glass.  Attempts  were  made  to 
use  this  glass  for  chemical  instruments,  but  without  much 
success  until  1899.  It  proved  to  be  permeable  to  gases, 
and  so  cannot  be  used  for  thermometers. 

Silica  Threads  were  obtained  by  Boys  in  1887  from 
melted  silica,  and  were  used  for  suspension  of  galvanometer 
needles.  These  threads  offer  two  marked  advantages  over 
ordinary  glass  threads.  The  residual  torsion  of  melted 
silica  threads  is  very  weak,  the  threads  returning  to  zero 
immediately  even  after  considerable  torsion,  whereas  with 
other  substances  the  return  is  made  only  very  slowly.  Again, 
when  conducting  threads  are  required,  the  greater  infusibility 
of  silica  permits  the  use  of  hot  platinizing  or  gilding,  which 
is  much  stronger  than  the  cold  silvering  alone  possible  on 
glass.  These  superficially  conducting  silica  filaments  have 
been  used  by  Binthoven  for  making  extremely  sensitive 
galvanometers.  The  filament  is  stretched  vertically  between 
the  two  poles  of  a  powerful  electro-magnet ;  its  surface, 
silvered  and  illuminated  laterally  by  an  arc  lamp,  the  light 
from  which  is  reflected  on  the  generatrix  at  45°  from  the 
cylindrical  filament,  gives  an  infinitely  thin  luminous  line. 
A  microscope  objective  magnifying  700  times  projects  the 
image  of  this  illuminated  generatrix  on  a  transparent  screen 
or  on  a  photographic  plate  animated  by  a  movement  of 
translation. 

Fused  Silica  is  also  used  for  making  photographic 
objectives.  Crystallized  quartz  has  remarkable  trans- 
parency for  ultra-violet  radiation,  and  the  same  property 
may  be  expected  in  fused  silica  with  less  inconvenience  from 
double  refraction.  Fused  silica  is  a  non-conductor  of 
electricity  even  in  a  moist  atmosphere  and  at  high 
temperatures. 

Fused  silica  in  considerable  quantity  was  first  made  by 
Moissan  in  an  electric  furnace,  and  it  has  since  become 
a  regular  commercial  product  in  England  since  1904.  It  is 
difficult  to  obtain  silica  in  a  really  fluid  condition  in  bulk, 
but  it  is  much  more  readily  obtainable  in  a  plastic  condition 


44  SILICA   AND   THE  SILICATES 

in  which  it  can  be  worked,  and  much  of  the  silica  glass  pro- 
ducts on  the  market  are  produced  in  this  way.  The  tem- 
perature at  which  carborundum  is  formed  is  very  close  to 
the  temperature  needed  for  working  silica,  but  the  relatively 
easy  accurate  measurements  with  an  electric  furnace  make  it 
possible  to  control  the  temperature.  A  hollow  cylinder  of 
plastic  silica  is  formed  by  heating  a  mass  of  sand  round  the 
central  core  through  which  the  current  is  passed.  The 
heating  core  being  then  withdrawn,  the  hollow  cylinder  is 
removed  from  the  furnace  and  drawn  into  tubing.  For 
moulded  articles,  such  as  a  pipe,  the  hollow  plastic  cylinder 
is  closed  at  one  end  by  mechanical  pressure,  and  into  the 
other  end  is  pressed  a  refractory  nozzle  for  delivering  com- 
pressed air  ;  the  softened  mass  is  then  placed  in  the  mould, 
and  compressed  air  applied  to  give  the  shape.  More  than 
200  Ibs.  of  the  softened  material  can  be  worked.  In  the 
plastic  condition  the  silica  is  extremely  ductile,  and  can  be 
drawn  out  like  glass  in  lengths  of  90  to  100  ft.  Articles 
made  as  described  have  a  rough  external  surface,  but  small 
laboratory  ware  is  ground  and  finally  glazed  by  melting  the 
surface  (electrically  or  by  means  of  the  oxy hydrogen  blow- 
pipe). Transparency  is  impaired  by  the  air-bubbles  asso- 
ciated with  sand,  which  are  very  difficult  to  remove  from  the 
very  viscous  plastic  silica.  At  1200°  C.  devitrification 
commences  in  fused  silica,  so  that  it  cannot  be  used  for 
continuous  work  at  higher  temperatures.  It  is  quite 
satisfactory  for  pyrometric  measurements,  etc. 

Transparent  Fused  Silica. — Herschkowitsch  first  made 
fused  silica  optically  homogeneous,  and  exhibited  specimens 
in  Paris  in  1900.  The  great  difficulty  in  accomplishing  this 
arises  from  an  allotropic  transformation  at  575°  C.,  accom- 
panied by  sudden  and  considerable  expansion,  which  causes 
repeated  shivering,  and  after  re-melting  air  bubbles  are 
imprisoned.  Herschkowitsch  heated  quartz  crystals  to 
about  550°  C.,  then  suddenly  transferred  the  material  to  an 
electric  furnace  to  melt  quickly.  Heraeus,  in  Germany, 
about  the  same  time  obtained  transparent  silica,  though  not 
so  good.  Bredel  in  1905  obtained  fused  silica  without 


SILICA  45 

bubbles  by  a  process  depending  on  the  permeability  of 
silica  to  hydrogen. 

Transparent  fused  silica  is  now  made  in  England. 

Optical  Properties. — The  refractive  index  for  fused 
silica  is  for  the  D  line  1*4585,  being  lower  than  that  for 
quartz,  but  its  transparency  for  ultra-violet  rays  is  less 
than  that  of  quartz,  though  it  is  sufficiently  transparent 
for  all  rays  active  on  the  ordinary  photographic  plate. 

Specific  Gravity  of  fused  silica  is  2*204  (about  2*07 
when  made  from  sand)  as  against  2*651  for  quartz  and 
2*330  for  cristobalite  produced  by  transformation  of  quartz 
on  heating. 

Expansion  is  very  irregular  for  all  varieties  of  crystallized 
silica.  Amorphous  (fused)  silica  has  extremely  regular 
expansion,  and  extraordinarily  low  coefficient  of  expansion. 
Silica  bricks  which  were  quickly  heated  to  1650°  C.  showed 
very  regular  expansion,  only  one-third  that  of  quartz,  but 
this  was  not  pure  silica. 

The  mean  coefficient  of  expansion  of  amorphous  silica 
between  o°  and  1000°  is  given  by  Le  Chatelier  as  70  Xio~8, 
and  by  Holborn  and  Henning  as  54Xio~8.  The  coefficient 
of  expansion  of  this  amorphous  silica  is  sensibly  constant  at 
all  temperatures.  A  crucible  heated  to  bright  redness  can 
be  plunged  into  cold  water  without  risk. 

LITERATURE. 

(1)  "  La  Silice  et  Les  Silicates,"  H.  Le  Chatelier.     1914. 

(2)  "  Dictionary  of  Applied  Chemistry,"  edited  by  Thorpe,   5  vols. 
(Chiefly  articles  by  L.  J.  Spencer  and  the  late  F.  W.  Rudler.) 

(3)  Encyclopedia  Britannica,    nth    edition.     1911.     (Chiefly    articles 
by  L.  J.  Spencer,  the  late  F.  W.  Rudler,  and  Dr.  J.  S.  Flett.) 

(4)  Encyclopedia  Britannica,  gth  edition.     (Article  on  "  Mineralogy  " 
by  the  late  Dr.  M.  F.  Heddle.) 

(5)  "  The  Data  of  Geo-Chemistry,"  F.  W.  Clarke,  3rd  edition.     1916. 
The  small  "  Introduction  to  the  Study  of  Minerals,"  sold  at  the  British 

Museum  of  Natural  History  is  a  very  useful  little  work. 


SECTION  IL-SILICATES 

SIUCATES,  like  silica  itself,  easily  preserve  the  vitreous 
state,  that  is,  the  state  of  an  amorphous  solid,  and  some  of 
them  have  never  been  obtained  in  the  crystallized  state, 
others  only  with  difficulty.  Many  natural  crystallized 
silicates  cannot  be  reproduced  in  crystalline  condition  by 
melting  mixtures  containing  their  components  in  proper 
proportions,  and  then  cooling,  the  products  of  such  action 
being  only  glass. 

The  destruction,  by  heat,  of  the  crystalline  condition  of 
a  silicate  does  riot  always  result  in  the  immediate  production 
of  a  homogeneous  and  transparent  substance — that  is,  a 
glass.  Felspar,  for  example,  between  1100°  and  1200°  C. 
gives  a  white  material  resembling  porcelain  or  devitrified 
glass  in  appearance.  The  felspar  has  really  become  de- 
composed, with  formation  of  microscopic  crystals  of  silica. 
At  a  higher  temperature  the  material  becomes  completely 
vitreous  and  transparent,  and  remains  so  unless  cooled  too 
slowly.  Felspar  is  employed  in  this  way  in  the  manufacture 
of  transparent  pearls. 

The  tendency  of  silicates  to  keep  the  vitreous  state  is 
increased  by  mixing  several  silicates  together.  Thus,  glasses 
are  mixtures  of  silica  and  silicates,  and  the  tendency  of  such 
mixtures  to  crystallize  is  much  less  than  that  of  silica  or 
individual  silicates.  Silicates  of  sodium  and  magnesium 
can  be  easily  crystallized  separately,  but  a  mixture  of  one 
with  20  per  cent,  of  the  other  will  not  crystallize  at  all. 

Complexity  is  a  very  common  characteristic  of  silicates. 
Crystals  which  under  the  microscope  seem  absolutely 
homogeneous  show  very  great  variations  of  chemical  com- 
position. Felspars,  for  example,  contain  potassium,  sodium, 

46 


SILICATES  47 

and  calcium  in  very  variable  proportions,  and  there  is  also 
wide  variation  in  the  relation  of  the  quantity  of  silica  to  the 
total  of  the  basic  oxides.  So  also  the  pyroxenes,  whose 
chief  components  are  silica  and  magnesia,  include  varying 
amounts  of  iron  oxides,  lime,  and  sometimes  alumina  and 
other  metallic  oxides.  Micas  are  more  complex  still,  though 
retaining  the  same  crystalline  form  and  optical  properties, 
etc. 

Mitscherlich's  Law. — It  was  supposed  that  only  chemical 
elements  or  definite  compounds  formed  crystalline  bodies. 
Amorphous  substances  in  many  cases  were  homogeneous 
mixtures  of  different  kinds  of  matter,  and  in  the  case  of 
solids  like  glass  are  considered  to  be  solid  solutions.  But 
substances  in  the  crystallized  state  were  found  to  show 
distinct  variations  in  the  composition  of  precisely  similar 
crystals.  Mitscherlich  concluded  that  identity  of  chemical 
constitution  and  identity  of  crystalline  form  were  equally 
indispensable  for  the  formation  of  crystals  of  variable 
composition,  and  enunciated  the  relationship  in  a  law  which 
bears  his  name.  He  called  these  mixtures  of  crystallized 
bodies  isomorphous  mixtures. 

Without  going  further  into  this  question  of  isomorphous 
mixtures,  it  may  be  stated  that  though  the  conditions  of 
identity  of  crystalline  form  and  of  chemical  constitution 
facilitate  the  formation  of  mixed  crystals,  they  are  not 
indispensable.  It  is  better,  therefore,  in  general  to  use  the 
expression  "  mixed  crystals  "  or  solid  solution  rather  than 
isomorphous  mixture  ;  solid  solution  includes  both  crystal- 
line and  amorphous  mixtures,  and  has  the  advantage  of 
including  mixtures  which  obey  certain  common  laws  of 
chemical  mechanics,  irrespective  of  the  presence  or  absence 
of  crystallization. 

Solid  Solutions  in  Minerals. — These  were  first  studied 
accurately  by  Tschermak,  in  the  case  of  the  felspars. 
Tschermak  concluded  that  the  felspars  form  a  continuous 
series,  ranging  between  the  two  extreme  members, 
the  felspar  orthoclase  or  albite,  BSiC^.AlgOg.KgO  or 
6SiO2.Al2O3.Na2O,andthefelsparanorthite,2SiO2.Al2O3.CaO, 


48  SILICA   AND  THE  SILICATES 

the  chemical  constitution  of  the  latter  being  obviously  quite 
different  from  that  of  the  alkali  felspar.  This  conception 
agrees  with  the  fact  that  in  chemical  analyses  of  felspars 
an  increase  in  lime  is  accompanied  by  a  proportionate  de- 
crease in  silica.  Though  at  first  strongly  opposed  in  some 
quarters,  Tschermak's  views  are  now  generally  accepted. 

Constitution  of  Mixed  Silicates. — The  existence  of 
mixed  crystals  of  different  silicates  greatly  facilitates  an 
understanding  of  the  constitution  of  natural  silicates  of 
variable  composition.  They  are  really  mixtures  of  a  limited 
number  of  relatively  simple  compounds,  and  the  problem  of 
the  constitution  of  silicates  is  the  determination  of  the 
comparatively  few  compounds  which  when  mixed  together 
give  rise  to  the  extensive  series  of  natural  silicates. 

As  regards  the  felspars,  the  problem  is  simple,  because 
all  the  intermediate  stages  are  known  between  the  two 
definite  compounds  albite  and  anorthite.  To  verify  the 
accuracy  of  the  constitution  attributed  to  mixed  crystals, 
it  is  enough  to  establish  that  the  numerical  relation  between 
the  quantities  of  silica,  lime,  and  alkaline  bases  is  in  agree- 
ment with  what  the  formulae  of  the  two  extreme  compounds 
(albite  and  anorthite)  require.  That  is,  for  one  molecular 
proportion  of  alumina  (A12O8)  the  molecular  proportion  of 
silica  (SiO2)  will  be  equal  to  twice  the  number  of  molecular 
proportions  of  lime  (CaO)  plus  six  times  the  number  of 
molecular  proportions  of  alkaline  bases  (Na2O,  K2O). 
This  is  illustrated  by  the  following  composition  of  a  felspar 
given  by  Rammelsberg  : 

Percentage  Molecular  Molecules  for 

Composition.  Composition.  iA!2O3. 

Silica       ..  ..  58-32  1-94  372 

Alumina..  ..  26-52  0*52  roo 

Lime        . .  . .  8'i8  0*29  0-56 

Soda        . .  . .  6*16  0*17  0*33 

Potash     . .  . .  2*36  0-05  o'io 

The  total  of  the  lime,  soda,  and  potash  is  0-99  (practically 
ro)  to  i  of  alumina,  as  would  be  required  for  the  formula 
of  felspar ;  the  corresponding  proportion  of  silica  according 


SILICATES  49 

to  Tschermak's  law  is  0*56  X2  +0*43  X 6  =3 70,  practically 
identical  with  the  3*72. 

With  the  pyroxenes  the  problem  is  more  complex, 
mainly  owing  to  the  presence  in  some  of  them  of  alumina, 
and  apparently  the  silicate  of  magnesia  crystallizes  with 
alumina  in  various  proportions ;  this,  however,  is  highly 
improbable,  and  has  not  been  seriously  suggested.  It  is 
admitted  that  the  simple  silicate  of  magnesia  can  crystallize 
with  an  alumino-silicate  of  magnesia  (of  rare  occurrence  in 
the  separate  condition  as  the  mineral  kornerupine)  of  the 
formula  SiO3.Al2O3.MgO,  or  with  garnet,  3SiO2.Al2O3.3MgO. 
This  hypothesis  would  readily  explain  the  composition  of 
aluminous  pyroxene,  but  other  hypotheses  would  be  equally 
satisfactory  for  that  purpose,  and  there  is  not  a  continuous 
series  (as  in  the  felspars)  from  pyroxene  to  kornerupine,  a 
considerable  gap  occurring  between  the  latter  and  the 
pyroxene  minerals  with  most  alumina. 

In  micas  the  problem  is  more  complicated  owing  to  the 
presence  of  water,  but  certain  micas  (as  white  mica  or 
muscovite)  have  a  relatively  simple  composition  and  corre- 
spond to  definite  compounds. 

Other  silicates — such  as  tourmaline,  which  occurs  in 
very  large  homogeneous  crystals — have  very  complicated 
and  variable  compositions. 

Many  silicates  contain  combined  water,  some  of  which 
by  analogy  with  other  salts  (phosphates,  bicarbonates, 
bisulphates,  etc.)  is  regarded  as  water  of  constitution,  and 
another  portion,  usually  more  weakly  combined,  is  considered 
to  be  water  of  hydration.  It  is,  however,  not  always  easy 
to  distinguish  water  of  hydration  from  water  of  constitution. 
Many  hydrated  salts  which  lose  their  water  of  hydration  on 
heating  can  take  it  up  again.  With  hydrated  silicates  this 
happens  only  with  a  few  alkaline  and  alkaline  earthy  silicates. 

In  some  silicates  which  only  lose  their  water  at  a  very 
high  temperature,  as  kaolinite  and  apophyllite,  the  water  is 
considered  as  water  of  constitution. 

Silicates  as  a  rule  do  not  easily  enter  into  chemical 
reaction  like  ordinary  soluble  salts.  Hydrofluoric  acid 
c.  4 


50  SILICA   AND   THE  SILICATES 

destroys  silicates  and  silica  by  forming  silicon  fluoride, 
which  escapes  in  the  form  of  a  gas.  Fusion  with  alkalies  or 
alkaline  carbonates  also  completely  destroys  silicates  by 
converting  them  into  silicates  of  the  alkali  metals. 

Silicic  Acids. — From  analogies  with  organic  compounds 
a  normal  silicic  acid  or  ortho silicic  acid  is  presumed  to 
exist  of  the  composition  represented  by  the  formula  Si(OH)4 
or  H4SiO4  or  SiO2.2H2O.  It  has  never  been  produced, 
but  normal  esters  are  known,  like  ethyl  orthosilicate, 
(C2H5)4SiO4,  and  many  metallic  silicates  corresponding  to 
it  occur,  as  olivine,  Mg2SiO4  or  SiO2.2MgO,  fayalite,  Fe2SiO4 
or  SiO2.2FeO. 

Removal  of  a  molecule  of  water  from  orthosilicic  acid 
would  leave metasilicic  acid,SiO(OH)2or  H2SiO3  or  SiO2.H2O. 
In  this  case  also  the  acid  itself  is  unknown,  but  many  silicates 
corresponding  to  it  are  known,  as  wollastonite,  CaSiO3  or 
SiO2.CaO,  enstatite,  MgSiO3  or  SiO2.MgO  ;  no  esters  are 
known. 

From  these  two  acids,  other  acids  are  derived  by  poly- 
merization and  removal  of  water  : 

Diorthosilicic  acid,  Si2O(OH)6  or  H6Si2O7  or  2SiO2.3H2O. 
Dimetasilicic  acid,  Si2O3(OH)2  or  H2Si2O5  or  2SiO2.H2O. 

In  this  way  an  indefinite  series  of  acids  could  be  derived 
theoretically. 

Classification  of  Silicates. — For  different  purposes, 
silicates  have  been  classified  in  various  ways.  Mineralogists 
and  geologists  classify  minerals  in  accordance  with  their 
crystalline  forms  and  their  power  of  crystallizing  together 
mixed  in  various  proportions — that  is,  of  forming  solid 
solutions  of  mixed  crystals.  This  property  of  silicates,  of 
mixing  thus,  notwithstanding  very  great  differences  of 
composition,  forming  very  fine  and  apparently  more  or  less 
homogeneous  crystals,  is  shared  only  with  metallic  alloys. 
Geologists  lay  special  stress  on  the  occurrence  of  the  minerals 
in  the  same  rocks.  Thus  in  granites  and  similar  rocks 
felspar  is  often  associated  with  mica  and  kaolinite,  so  these 
three  minerals  are  classed  together  by  geologists. 


SILICATES  51 

Chemists  have  classified  silicates  in  several  distinct  ways, 
but  in  most  of  them  difficulties  have  arisen  in  connection 
with  the  silicates  which  contain  alumina.  Vernadsky 
proposed  to  overcome  those  difficulties  by  assuming  the 
existence  of  complex  alumino-silicic  acids,  comparable  to 
the  thoroughly  investigated  silico-tungstic  and  silico- 
molybdic  acids.  Many  authorities  have  adopted  this  view, 
and  it  seems  to  afford  the  readiest  means  of  showing  the 
relationships  existing  among  the  silicates,  based  on  the 
relation  between  the  silica  and  alumina.  This  must,  however, 
be  combined  with  the  mineralogist's  classification  based  on 
crystalline  forms,  which  brings  together  silicates  having  the 
property  of  forming  mixed  crystals,  as  this  classification  is 
certainly  the  best  for  the  rock-forming  minerals  of  the  earth's 
crust. 

Others  who  have  sought  to  elucidate  the  constitution  of 
silicates  include  F.  W.  Clarke,  who  made  a  keen  study  of 
the  subject,  and  W.  and  D.  Asch,  who  assumed  the  existence 
of  closed  rings  consisting  of  six  or  five  groups  of  SiO2,  or  of 
half  (A12O3),  connected  with  one  another  and  with  different 
elements  and  radicles  in  various  ways.  Unfortunately, 
though  highly  ingenious,  these  can  only  be  regarded  as 
speculative  attempts  to  explain  the  facts,  and  considerations 
of  space  preclude  further  attention  to  them  here. 

Non-aluminous  Silicates  include  dibasic  silicates,  mono- 
basic silicates,  and  sundry  silicates. 

Dibasic  non-aluminous  Silicates  or  Orthosilicates. 
— The  most  important  member  of  this  group  is  olivine,  con- 
sisting of  magnesium  orthosilicate,  SiO2.2MgO  or  Mg2SiO4, 
with  ferrous  silicate  and  other  isomorphous  silicates  of  the 
same  family,  partly  replacing  the  magnesium  silicate  ;  it 
constitutes  an  essential  component  of  the  most  basic  rocks. 
The  name  "  olivine  "  was  given  in  allusion  to  the  olive-green 
colour  often  possessed  by  the  mineral.  The  hardness  is 
between  6  and  7,  and  the  specific  gravity  about  3*3.  The 
refractive  power  and  double  refraction  are  higher  than  in 
many  other  minerals.  Olivine  occurs  usually  as  compact 
or  granular  masses,  or  as  grains  and  blebs  in  certain  basic 


52  SILICA  AND  THE  SILICATES 

igneous  rocks,  including  basalt  and  dolerite,  diabase,  gabbro, 
peridotite,  etc.     It  also  occurs  in  meteorites. 

Hot  hydrochloric  acid  decomposes  olivine,  gelatinous 
silica  being  separated.  Olivine  rather  readily  changes  under 
the  influences  of  atmospheric  and  other  natural  agencies 
into  serpentine,  which  is  a  hydrated  magnesium  silicate. 

Transparent  varieties  of  olivine  are  used  in  jewellery 
under  the  names  of  chrysolite  and  peridot  (or  peridote),  but 
various  green  stones  are  incorrectly  called  olivine  by 
jewellers.  The  name  chrysolite  is  sometimes  restricted  to 
pale  yellowish-green  transparent  olivine,  and  the  name 
peridot  to  darker  and  decidedly  green  kinds.  Being  of 
inferior  hardness  the  polished  stone  is  readily  abraded  by 
wear.  The  final  polishing  of  olivine  is  effected  on  a  copper 
wheel  moistened  with  sulphuric  acid.  Most  of  the  peridot 
of  commerce  is  from  an  island  in  the  Red  Sea.  Transparent 
pebbles  of  olivine,  suitable  for  cutting,  are  found  in  Arizona, 
Montana,  and  New  Mexico,  and  from  their  shape  and  pitted 
surface  they  are  called  "  Job's  tears." 

Olivine  in  its  different  varieties  contains  from  about 
5  to  30  per  cent,  of  ferrous  oxide,  the  proportion  in  the 
varieties  used  as  gems  being  about  9  per  cent.  When 
ferrous  oxide  is  less  than  5  per  cent,  the  colourless  or  yellowish 
mineral  is  forsterite,  SiO2.2MgO  or  Mg2SiO4,  which  is  found 
in  many  crystalline  limestones.  Hyalosiderite  contains 
over  25  per  cent,  ferrous  oxide.  On  the  other  hand,  with 
ferrous  oxide  practically  the  only  base  present,  the  mineral 
is>  fayalite,  SiO2.2FeO  or  Fe^iO^  which  is  dark  brown  or 
black  ;  it  occurs  in  the  Mourne  Mountains  and  elsewhere  in 
the  North  of  Ireland,  at  Fayal  in  the  Azores  (whence  the 
name),  and  also  in  crystalline  iron  slags. 

When  the  basic  constituents  consist  essentially  of 
magnesia  and  lime  in  molecular  proportions  the  rare  mineral 
monticellite,  SiO2.MgO.CaO  or  CaMgSiO4,  is  formed;  it 
occurs  as  yellowish-grey  crystals  and  grains  in  limestone  at 
Monte  Somma  (Vesuvius). 

All  the  minerals  mentioned  are  isomorphous  with  olivine. 

Calcium  Orthosilicate  or  Dicalcium  Silicate,SiO2.2CaO 


SILICATES  53 

or  Ca2SiO4,  is  not  known  to  occur  in  nature,  but  is  of  im- 
portance in  connection  with  some  industries,  including 
hydraulic  limes  and  cements,  metallurgy  (blast  furnace 
slags),  etc.  It  was  obtained  by  Le  Chatelier  by  heating  a 
mixture  of  its  components  to  nearly  1600°  C.  It  has  the 
property  of  falling  into  a  white  impalpable  powder  on  cooling. 
The  white  powder  as  seen  under  the  microscope  consists  of 
prismatic  fragments.  This  spontaneous  pulverization  has 
been  observed  with  all  blast  furnace  slags  sufficiently 
calcareous,  and  has  been  attributed  to  atmospheric  moisture 
causing  slaking  of  the  slags  by  hydration,  as  happens  with 
lime.  Similarly  pieces  of  burned  cement,  drawn  hot  from 
the  kiln,  are  pulverized  after  a  time.  This  transformation 
is  accompanied  by  disengagement  of  heat.  This  dust  has 
very  little  value  as  cement,  and  represents  serious  loss  in 
manufacture.  L,e  Chatelier  has  established  that  this  property 
belongs  to  SiO2.2CaO  only. 

The  mixture  SiO2+i*5CaO  gives  fragments  not 
pulverized.  Pulverization  is  diminished  by  replacing  part 
of  the  lime  of  SiO2.2CaO  by  magnesia.  Addition  of  alumina 
and  iron  oxide  equally  diminishes  pulverization ;  it  is  thus 
that  blast  furnace  slags  often  take  several  days  to  become 
disintegrated  and  sometimes  give  only  a  coarse  sand  very 
different  from  the  impalpable  powder  produced  by  the  pure 
silicate.  The  explanation  is  similar  to  that  for  dimorphous 
potassium  sulphate. 

Unlike  the  metasilicate,  calcium  orthosilicate  is  readily 
decomposed  by  ammonium  salts,  hot  or  cold,  in  concen- 
trated or  dilute  solution.  Acids  act  still  more  rapidly,  but 
water  neither  directly  hydrates  nor  decomposes  it. 

The  true  melting  point  of  calcium  orthosilicate  is  2080°  C. 

In  constructing  cement  kilns,  cement  already  burned 
can  be  used  as  a  refractory,  which  is  softened  and  agglo- 
merated during  the  first  firing  ;  in  a  second  firing  it  no  longer 
melts  at  the  same  temperature. 

The  melted  silicate  crystallizes  at  2080°,  giving  a  variety 
a,  of  specific  gravity  3-27,  hardness  between  5  and  6,  crystal- 
lized in  the  monoclinic  system.  It  is  stable  down  to  1410°  C., 


54  SILICA   AND   THE  SILICATES 

being  then  transformed  (with  slow  cooling)  into  a  second 
variety  p.  By  sudden  cooling  from  a  temperature  above 
1410°  it  can  be  preserved  unchanged  to  ordinary  tempera- 
ture. Unlike  the  pulverulent  variety  of  calcium  ortho- 
silicate  obtained  by  slow  cooling,  it  exhibits  hydraulic 
properties.  Certain  very  calcareous  blast  furnace  slags  are 
transformed  into  true  cements  by  sudden  cooling.  Cements 
thus  tempered  do  not  fall  into  powder. 

The  superiority  of  Portland  cements  made  in  rotary 
kilns  is  due  to  the  same  cause.  In  the  ordinary  fixed  kilns 
the  cooling  of  the  cement  is  always  very  slow,  about  a  third 
of  the  cement  falls  into  powder  and  it  loses  at  the  same  time 
a  similar  proportion  of  its  hydraulic  properties.  Cement 
from  rotary  kilns,  going  out  from  the  hottest  region  of  the 
kiln,  gives  no  dust  and  preserves  its  full  hydraulic  properties. 

The  j8  variety  of  calcium  orthosilicate,  produced  by 
cooling  slowly  below  1410°  C.,  crystallizes  in  the  ortho- 
rhombic  system,  but  it  cannot  be  preserved  to  ordinary 
temperature.  At  675°  C.  the  variety  ft  is  transformed  into 
a  third  variety  y  with  great  increase  of  volume,  which  causes 
pulverization  of  the  whole  mass.  The  specific  gravity 
changes  from  3*280  to  2*974,  and  the  existence  of  three  easy 
cleavages  in  the  crystals  facilitates  complete  fragmentation. 
Even  slow  cooling  will  not  prevent  this  transformation,  and 
pulverization  can  only  be  avoided — at  least  with  pure 
silicate — by  tempering  from  a  temperature  above  1410°  C. 

In  the  presence  of  certain  foreign  matters,  calcium 
orthosilicate  can  apparently  be  preserved,  even  by  slow 
cooling,  under  its  hydraulic  form  a.  Specimens  of  hydraulic 
lime  and  of  cement  are  sometimes  found  closely  approaching 
the  composition  of  this  silicate,  and  not  pulverized.  The 
cause  was  not  at  first  recognized.  Candlot  showed  that 
artificial  cements  could  be  prepared  poor  in  lime,  which 
normally  would  be  completely  pulverized  on  cooling,  and 
would  have  no  hydraulic  properties,  by  adding  to  the 
mixture  a  certain  quantity  of  calcium  sulphate.  All  the 
natural  cements  which  harden  quickly,  generally  little 
charged  in  lime,  include  sulphate. 


SILICATES  55 

Disintegration  on  cooling  melted  mixtures  of  calcium 
silicates  and  lime  begins  with  a  lime  content  of  51  to  54 
per  cent.,  according  to  rapidity  of  cooling.  The  mixtures 
with  less  than  34  per  cent,  of  lime  remain  vitreous  ;  with 
between  34  and  47  per  cent,  of  lime,  fibres  of  pseudo- 
wollastonite  are  formed. 

Tri-calcium  Silicate. — Le  Chatelier  has  recognized  crys- 
tals closely  approaching  SiO2-3CaO  in  composition,  without 
any  iron,  but  only  silica  and  lime,  and  they  are  more  abun- 
dant as  the  iron  and  alumina  in  the  cement  is  less.  There  is 
no  free  lime  in  them,  for  in  presence  of  water  these  cements 
harden,  and  do  not  swell  like  lime,  whereas  with  addition  of 
only  0'5  per  cent,  of  free  lime  a  distinct  swelling  is  produced. 

It  is  possible  it  may  be  only  a  solid  solution  of  lime  and 
calcium  orthosilicate  with  the  lime  reaching  the  proportion 
3CaO  to  iSiO2.  It  is,  however,  established  that  the  essential 
constituent  of  all  hydraulic  products,  is  a  mixture  or  com- 
pound of  lime  and  silica,  containing  more  than  two  molecules 
lime  for  one  of  silica,  obviously  three  molecules. 

Hydrated  Silicate  of  Lime. — No  definite  hydrated 
silicate  of  lime  (calcium  silicate)  has  been  obtained  by  the 
direct  action  of  water  on  the  anhydrous  silicates.  Calcium 
metasilicate  does  not  hydrate,  and  the  other  calcium  silicates 
when  hydrated  are  at  the  same  time  decomposed,  lime  being 
liberated.  The  best  process  for  obtaining  hydrated  silicate 
is  the  action  of  colloidal  silica  on  lime-water,  when  a  very 
bulky  precipitate  is  obtained  ;  i  g.  of  this  substance  in 
suspension  in  water  occupies  about  two  litres.  In  presence 
of  almost  saturated  lime-water,  this  silicate  approaches  two 
molecules  of  lime  to  one  of  silica  ;  in  presence  of  lime-water 
diluted  with  100  volumes  of  water,  the  composition  is 
practically  one  molecule  of  lime  to  one  molecule  of  silica. 

The  hydration  of  tricalcium  silicate  in  presence  of  water, 
which  is  the  fundamental  reaction  in  the  hardening  of 
hydraulic  cements,  is  not  a  simple  phenomenon  of  hydration, 
since  in  presence  of  water  the  silicates  of  lime  cannot  retain 
more  than  i  '8  molecule  of  lime  for  i  of  silica.  In  fact,  it  is 
easy  to  recognize  in  the  hydration  of  cements  the  formation 


56  SILICA   AND   THE  SILICATES 

of  hydrate  of  lime  (calcium  hydroxide)  crystallized  in 
hexagonal  lamellae  large  enough  to  be  visible  to  the  naked 
eye,  if  precautions  have  been  taken  to  prevent  access  of 
atmospheric  carbonic  acid.  The  reaction  is  : 

SiO2.3CaO  +4'5H2O  =SiO2.CaO.2'5H2O  +2CaO.H2O 

Part  of  the  hydrate  is  crystallized,  and  part  fixed  by 
capillary  affinity  on  the  hydrated  silicate.  This  hydrated 
silicate  is  in  crystals  so  thin  that  the  strongest  magnification 
of  a  microscope  does  not  suffice  to  distinguish  them.  This 
expresses  L,e  Chatelier's  view  of  the  course  of  the  operation. 

Willemite,  SiO2.2ZnO  or  Zn2SiO4,  is  zinc  orthosilicate, 
and  has  been  worked  as  an  ore  of  zinc  in  a  few  places.  Wille- 
mite has  a  hardness  of  5j,  and  specific  gravity  about  4. 
It  is  colourless,  white,  greenish-yellow,  apple-green,  light 
red,  etc. 

Electric  Calamine  or  Smithsonite  is  a  hydrated  silicate 
of  this  class,  which  forms  an  ore  of  zinc. 

Phenacite  or  Phenakite,  SiO2.2BeO  or  Be2SiO4,  is  beryl- 
lium (or  glucinum)  orthosilicate.  It  is  sometimes  used  as 
an  ornamental  stone,  and  is  colourless  or  bright  yellow, 
transparent  or  opaque.  The  hardness  is  7|,  and  specific 
gravity  about  2*98.  At  first  sight  it  might  be  mistaken  for 
quartz — hence  the  name,  from  Greek  phenax,  "  a  deceiver  " — 
but  the  specific  gravity  is  considerably  higher.  When  well 
cut  and  polished,  phenacite  is  so  brilliant  that  it  may  some- 
times be  mistaken  for  diamond. 

Apophyllite,  which  may  be  noticed  briefly  here,  is 
a  peculiar  hydrated  silicate  of  lime  and  potash  (generally 
containing  a  little  fluorine),  with  a  lamellar  structure.  It  is 
transparent  or  opaque,  white  or  greyish,  sometimes  with  a 
tinge  of  green,  yellow,  or  red.  The  hardness  is  4!  to  5,  and 
specific  gravity  2*33.  Heated  before  the  blow-pipe,  it 
separates  into  thin  lamellse  (whence  the  name).  It  occurs 
at  several  places  in  Scotland  and  Ireland,  as  well  as  in  many 
foreign  localities.  It  is  occasionally  used  as  a  gemstone, 
and  is  then  termed  fish-eye  stone. 


SILICATES  57 


MONOBASIC  SIUCATES  OR  METASIUCATES. 

Sodium  Metasilicate,  SiO2.Na2O,  or  Na2SiO3  in  com- 
bination with  excess  of  silica,  forms  the  substance  well 
known  in  commerce  as  water-glass  or  soluble-glass,  which 
(when  solid)  is  glassy  in  appearance,  and  dissolves  fairly 
easily  in  hot  water. 

Sodium  metasilicate  is  obtained  by  melting  together 
molecular  proportions  of  silica  and  sodium  carbonate 
(60  to  106  parts  by  weight  of  approximately  pure  anhydrous 
materials).  The  compound  melts  at  1000°  C.,  and  crystal- 
lizes on  cooling ;  it  is  very  soluble  in  water,  but  often 
leaves  some  undissolved  silica  resulting  from  partial  de- 
composition. These  silicates  are  quite  stable  only  in 
presence  of  excess  of  free  alkali ;  by  evaporation  of  this 
solution,  various  crystallized  hydrated  silicates  are  obtained, 
differing  only  by  slight  quantities  of  water.  The  metasilicate 
is  the  only  knewn  crystalline  sodium  silicate,  the  others 
being  glassy  bodies.  Acids,  even  carbonic  acid,  decompose 
an  aqueous  solution,  gelatinous  silica  being  separated,  a 
result  which  is  also  effected  by  alkaline  carbonates  and 
chlorides,  especially  ammonium  chloride.  Salts  of  alkaline 
earths  give  insoluble  double  silicates. 

Commercial  Sodium  Silicate  or  Soluble  Glass  is 
obtained  by  melting  one  molecular  proportion  of  sodium 
carbonate  with  from  three  to  four  molecular  proportions  of 
silica ;  it  is  vitreous  on  cooling.  Na2O.4SiO2  contains 
about  79  per  cent,  of  silica. 

These  alkaline  glasses  when  powdered  and  treated  with 
water  .dissolve  completely  when  less  than  three  molecules 
of  silica  are  present  to  each  molecule  of  soda ;  otherwise 
part  of  the  silica  generally  remains  undissolved. 

On  treating  melted  soluble  glass — corresponding  to 
3SiO2.Na2O  or  thereabouts — with  sufficient  water  to  give 
a  solution  of  specific  gravity  1*16,  the  compound  2SiO2.Na2O 
(or  NaaSiO5)  is  found  in  the  solution,  leaving  a  residue  of 
insoluble  silica.  Concentration  of  the  solution  to  a  density 


58  SILICA   AND   THE  SILICATES 

of  1*53  results  in  precipitation  of  more  silica,  the  definite 
silicate  SiO2Na2O  (or  Na2SiO3)  being  left  in  solution. 

The  addition  of  ammonia  to  silicate  of  soda  solution 
produces  a  precipitate  which  is  redissolved  on  boiling. 

Industrially,  silicate  of  soda  is  prepared  by  wet  and  dry 
methods :  (i)  by  melting  silicious  sand  with  sodium 
carbonate,  (2)  by  substituting  a  mixture  of  sodium  sulphate 
and  carbon  for  the  sodium  carbonate  in  the  preceding 
method,  (3)  by  the  action  of  water  vapour  on  a  strongly 
heated  mixture  of  sand  and  sodium  chloride,  and  (4)  in 
solution  directly  by  heating  in  an  autoclave  lye-washings 
of  soda  with  ground  flint  or  infusorial  earth.  The  dry 
method  (i  or  2  above)  is  carried  out  in  a  Siemens  or  similar 
gas-fired  furnace  heated  at  about  1100°  C.  for  5  to  8  hours. 
Continuous  coal  fires  are  also  used  for  the  same  process, 
producing  1250  kilogs.  of  water-glass  in  24  hours  with  1500 
kilogs.  of  coal.  The  fused  product  is  run  into  an  iron 
receptacle  to  cool.  Typical  mixtures  for  the  charge  are  : 
28  Ib.  calcined  Na2CO3,  or  60  Ib.  calcined  sodium  sulphate 
and  15-20  Ib.  coal,  to  100  Ib.  powdered  quartz ;  or  no  Ib. 
sodium  sulphate  and  10  Ib.  coal,  or  100  Ib.  of  50  per  cent, 
sodium  carbonate  and  3  Ib.  coal  to  180  Ib.  white  sand  ;  the 
coal  assists  reduction. 

The  fused  product  is  broken  up  by  a  stone  breaker  or 
ground  by  suitable  machinery,  and  dissolved  in  water  by 
long  boiling,  best  under  pressure.  The  resulting  solution, 
clarified  by  standing,  is  evaporated  to  40°  Be.  in  iron  pans, 
100  Ib.  of  the  solid  yielding  about  300  Ib.  of  this  solution. 
Sulphide,  when  present,  may  be  removed  by  adding  some 
copper  scale  or  litharge  when  preparing  the  solution. 

The  wet  method  is  much  used  because  of  the  greater 
uniformity  of  the  product,  and  because  it  is  obtained  in 
solution  at  once.  Silica  (preferably  infusorial  earth)  is 
digested  with  a  solution  of  caustic  soda  of  specific  gravity 
1-22-1-24  (not  higher)  under  3  or  4  atmospheres  pressure. 
(The  caustic  soda  is  prepared  by  causticizing  a  solution  of 
sodium  carbonate  with  lime.)  The  liquid  is  heated  by  blow- 
ing in  steam,  and  is  kept  agitated  by  machinery .  Complete 


SILICATES  59 

solution  is  effected  in  about  3  hours  ;  it  is  indicated  when,  in 
a  sample  withdrawn,  the  suspended  matter  settles  quickly 
and  has  a  brick-red  colour  (Fe2O3),  and  the  liquid  has  no 
more  than  a  faint  alkaline  reaction.  Stronger  caustic  soda 
solution  holds  the  fine  sand  and  ferric  oxide  in  suspension 
so  long  that  the  liquid  would  not  be  properly  clarified  in 
several  days. 

The  clarified  liquid  is  drawn  off  and  concentrated  (usually 
to  a  specific  gravity  of  about  17)  in  iron  pans  ;  2'8  parts  of 
infusorial  earth  are  used  to  i  part  of  caustic  soda,  the 
resulting  solution  having  a  specific  gravity  of  about  i'i8 
(due  to  condensed  steam).  Powdered  flints,  quartz,  etc., 
could  be  used,  but  would  need  longer  digestion  and  under 
7  to  8  atmospheres  pressure. 

A  pure  silicate  may  be  made  from  the  crude  product  by 
passing  a  current  of  carbon  dioxide  through  the  solution, 
filtering  off  the  precipitated  hydrated  silica,  and  redissolving 
it  in  caustic  soda. 

The  aqueous  solution  of  specific  gravity  17  usually 
contains  32  to  33  per  cent,  silica,  16  to  i6|  per  cent,  soda, 
2j  to  3  per  cent,  other  sodium  salts,  and  48  to  49  per 
cent,  water. 

Commercial  water-glass  is  usually  brownish  or  greenish, 
but  sometimes  colourless.  It  is  nearly  insoluble  in  cold 
water,  but  dissolves  completely  (though  slowly)  in  boiling 
water,  the  solubility  decreasing  with  increase  of  silica. 
Silicates  richer  in  alkali  than  Na2O.2'25SiO2  are  deliquescent 
and  too  poor  in  silica  for  ordinary  uses  of  water-glass. 

A  water-glass  known  as  "  double  soluble-glass  "  is  pre- 
pared by  using,  instead  of  sodium  carbonate,  sodium  and 
potassium  carbonates  in  quantities  to  give  about  equal 
proportions  of  sodium  and  potassium  silicates.  It  is  said 
to  give  a  thinner  solution  than  either  silicate  of  corresponding 
strength. 

Potassium  Silicates  are  prepared  like  sodium  silicates 
by  simply  substituting  potassium  carbonate  (or  sulphate) 
for  the  sodium  compound.  They  furnish,  on  cooling,  an 
uncrystallizable  glass,  even  more  soluble  and  deliquescent 


60  SILICA   AND  THE  SILICATES 

than  the  corresponding  sodium  silicate.  The  preparation 
of  hydrated  silicates  is  more  difficult  than  in  the  case  of 
sodium  silicates. 

Potassium  silicates  are  soluble  in  water  up  to  four 
molecular  proportions  of  silica  to  one  of  potash.  Solution 
of  soluble  potash  glass  of  specific  gravity  1*25  includes 
28  per  cent,  of  the  soluble-glass ;  when  made  into  a  paste 
with  chalk  or  zinc  oxide  it  gives  cements  which  soon  become 
very  hard,  adhering  to  glass  and  porcelain,  and  so  are  used 
for  repairing  broken  domestic  articles.  A  mixture  of 
2  parts  of  fluorspar  and  i  part  powdered  glass  made  into  a 
thick  paste  with  water-glass  is  used  as  a  cement  for  glass 
and  china. 

Solutions  of  soluble-glass  give  by  desiccation  a  transparent 
vitreous  mass  which  is  firm  and  elastic  without  being  deli- 
quescent. Hence  their  employment  for  hardening  the  linen 
bandages  on  fractured  limbs,  for  making  very  resistant 
junctions  between  pieces  of  chemical  apparatus,  after 
mixing  with  kaolin  or  asbestos,  for  coating  paper  labels  on 
glass  so  as  to  resist  washing  with  water ;  it  is  also  some- 
times used  for  rendering  theatre  decorations  incombustible. 

Sodium  silicate  is  largely  used,  owing  to  its  detergent 
properties,  as  an  addition  to  soap  (chiefly  toilet  soap),  to 
which  it  imparts  hardness  and  durability.  Two  typical 
analyses  of  such  soaps  show  : 

Soda.  Silica.  Water.  Fatty  Acids. 

I  .  .       12  10  30'0  48-0 

II  ,.     12-5  8-5  33-0  46-0 

Water-glass  is  used  as  a  fixing  agent  for  pigments  in 
the  calico-printing  and  dyeing  industries,  and  in  printing 
indigo  ;  also  as  a  resist  for  certain  colours,  and  in  the 
finishing  of  cotton  goods.  It  is  a  component  of  certain 
leadless  enamels  for  glazing  pans,  etc.,  for  domestic  use,  and 
is  sometimes  used  in  the  basic  linings  of  Bessemer  converters. 
Other  uses  are  for  fixing  fresco-painting,  for  rendering 
wood,  linen,  and  paper,  etc.,  non-inflammable,  for  bleaching 
jute,  for  preventing  wood  from  rotting,  and  for  preserving 


SILICATES  61 

eggs.  "  Artificial  vegetation  "  (so-called)  results  from  drop- 
ping small  pieces  of  certain  salts  (cobalt  chloride,  ferrous 
sulphate,  and  many  others)  into  a  solution  of  sodium  silicate, 
when  frond-like  appearances  are  produced,  due  to  the 
formation  of  insoluble  metallic  silicates. 

Sodium  silicate  is  much  used  for  impregnating  sandstone 
and  other  porous  stones  as  a  protection  against  weathering. 
The  stone  is  first  treated  with  a  solution  of  water-glass,  then 
with  a  solution  of  calcium  chloride  or  aluminium  sulphate, 
which  causes  the  deposition  of  an  insoluble  silicate  in  the 
pores  of  the  stone,  greatly  increasing  its  hardness  and 
durability.  The  soluble  sodium  salt  formed  is  removed 
by  a  subsequent  washing. 

Water-glass  plays  an  important  part  in  the  manufacture 
of  artificial  stone.  The  following  process  is  used  at  an 
English  works  for  making  chimney  pots,  mouldings,  etc. 
The  sand  or  sandstone  used  is  ground  wet  and  passed 
through  a  40 -mesh  sieve ;  it  is  then  thoroughly  dried  and 
mixed  with  enough  water-glass  solution  in  a  revolving  pan 
provided  with  scrapers  and  edge  mixers  to  form  a  moist 
coherent  mass.  It  is  next  pressed  into  moulds  and  before 
removal  is  saturated  with  a  solution  of  calcium  chloride 
(prepared  by  running  hydrochloric  acid  over  calcspar) 
drawn  through  by  a  vacuum  equal  to  about  20  inches  of 
mercury,  calcium  silicate  being  thus  formed.  The  stones 
are  transferred  to  a  boiling  tank  heated  by  steam,  and  after 
the  setting  is  completed  are  conveyed  to  washing  tanks,  in 
which  they  are  subjected  to  the  action  of  running  water  for 
a  fortnight,  to  remove  all  the  sodium  chloride. 

Sand  of  any  origin  and  size  of  grain,  gravel,  clay  calcined 
or  uncalcined,  granite  fragments,  etc.,  mixed  with  little 
more  than  10  per  cent,  of  highly  silicious  alkali  silicate 
melt  and  pressed  together,  combine  in  superheated  steam 
to  considerable  hardness,  which  does  not  differ  materially 
from  that  of  natural  sandstone,  clay,  slate,  etc.  Of  the 
artificial  stone  samples  thus  obtained,  only  those  cemented 
with  potassium  silicate  are  durable  in  water,  and  when 
fixed  for  a  time  some  potassium  disilicate  (K2Si2O6)  is  given 


62  SILICA   AND   THE  SILICATES 

up,  the  alkali  being  thus  gradually  lost,  but  nothing  is  lost 
in  hardness.  Those  obtained  with  sodium  silicate  melts, 
on  the  contrary,  in  spite  of  originally  greater  hardness, 
decompose  completely  in  water  within  a  short  time. 

Stereochromy,  or  the  art  of  mural  and  monumental 
-painting,  is  a  special  application  of  water-glass.  A  suitable 
ground  of  stone  or  cement  is  first  coated  with  lime-mortar 
to  form  the  under-ground  and  after  it  has  hardened  and 
become  thoroughly  dried,  water-glass  solution  is  allowed  to 
soak  well  into  the  mortar.  On  this  is  laid  the  over-ground, 
consisting  of  similar  constituents,  except  that  the  sand 
used  should  be  of  a  good  sharp  type,  and  that  a  thin  lye  of 
calcium  carbonate  is  added  to  the  mixture.  This  over- 
ground of  fine  cement  is  levelled,  and  when  dry  it  is  well 
soaked  with  water-glass  solution.  After  this  has  dried,  the 
painting  is  done  on  the  surface  in  water  colours.  The 
colours  are  finally  fixed  by  treating  with  water-glass,  prefer- 
ably a  mixed  potash  and  soda  water-glass.  Colours  used 
include  zinc  white,  chrome  and  some  other  greens,  chrome- 
red,  several  yellows  and  blues,  iron  oxide,  etc.  Such  paint- 
ings are  unaffected  by  rain,  smoke,  or  temperature  changes. 

Calcium  Metasilicate  or  monocalcium  silicate,  CaSiO3 
or  SiO2.CaO,  is  dimorphous.  It  occurs  naturally  (chiefly  in 
crystalline  limestones)  as  the  mineral  wollastonite  or  tabular 
spar,  the  latter  name  being  in  allusion  to  its  frequent  occur- 
rence in  flattened  crystals.  Its  hardness  is  5,  and  its  specific 
gravity  2*85.  It  is  formed  during  crystallization  (devitri- 
fication) of  certain  acid  blast  furnace  slags  and  ordinary 
glasses,  chiefly  bottle-glass  ;  in  these  cases  it  is  the  second 
form,  known  as  pseudowollastonite,  which  is  produced. 
Vitreous  calcium  metasilicate  is  easily  obtained  by  running 
the  melted  material  into  water.  This  glass  devitrifies 
rapidly  between  800°  and  1000°  C.,  forming  a  fibrous  mass 
of  crystals  of  wollastonite. 

THE  PYROXENES. 

Wollastonite  may  be  regarded  as  a  member  of  the 
pyroxene  family  of  minerals,  which,  excepting  only  felspars, 


SILICATES  63 

are  the  commonest  ingredients  of  nearly  all  igneous  rocks, 
especially  the  basic  rocks.  The  name  pyroxene  (from  Greek 
pur,  "fire,"  and  xenos,  "a.  stranger")  was  originally  given 
by  Haiiy  to  black  crystals  of  what  is  now  called  augite, 
occurring  in  lavas  of  Vesuvius  and  Etna,  because  of  the 
mistaken  assumption  that  they  had  got  into  the  lavas 
accidentally.  The  name  pyroxene  is  now  used  merely  as  a 
group  name. 

The  pyroxenes  crystallize  in  three  different  systems, 
but  they  all  show  distinct  cleavages  parallel  to  prism  faces, 
with  angles  between  them  of  about  87°  (and  93°) — the 
corresponding  cleavage  angle  of  the  amphiboles  (another 
family  of  minerals  showing  great  similarity  in  chemical 
composition  and  general  character  to  the  pyroxenes)  being 
about  56°  (and  124°).  The  composition  and  physical 
characters  vary  considerably  in  the  different  species. 

Apart  from  wollastonite — and  rhodonite,  the  correspond- 
ing manganese  silicate,  which,  as  the  name  implies,  has  a 
rose  colour,  and  which  is  sometimes  used  for  ornaments — 
the  simplest  member  of  the  pyroxene  group  is  enstatite, 
which  is  essentially  magnesium  metasilicate,  MgSiO3  or 
SiO2.MgO,  but  it  often  contains  a  little  iron  partly  replacing 
magnesium.  With  a  larger  proportion  of  iron  it  becomes 
bronzite,  and  with  still  more  iron  it  passes  into  hypersthene 
or  paulite.  Enstatite  (from  Greek  enstates,  "  an  opponent," 
because  it  is  nearly  infusible  before  the  blow-pipe)  and 
olivine  form  the  bulk  of  most  meteoric  stones.  It  is  white, 
greenish,  or  brown,  its  hardness  is  about  5j,  and  its  specific 
gravity  about  3*2.  Bronzite  is  a  green  or  brown  variety  of 
enstatite  containing  about  5  to  14  per  cent,  of  ferrous  oxide, 
with  a  bronze-like  lustre  (hence  the  name)  produced  by 
separation  of  very  fine  films  of  iron  oxide  and  hydroxide  on 
the  cleavage  surfaces.  I^e  Chatelier  ascribes  this  metallic 
lustre  to  the  presence  of  inclusions  of  titanium  compounds. 
Hypersthene  contains  15  to  30  per  cent,  of  ferrous  oxide,  and 
has  an  even  more  pronounced  metallic  sheen  than  bronzite. 
Bronzite  may  be  represented  as  (Mg,Fe)SiO3,  and  hyper- 
sthene as  (Fe,Mg)SiO3.  Bronzite  is  sometimes  cut  and 


64  SILICA   AND  THE  SILICATES 

polished  for  small  ornaments,  and  hypersthene  is  more 
extensively  used  in  the  same  way.  An  altered  form  of 
enstatite  or  bronzite  called  bastite  or  schiller-spar  is  hydrated 
and  has  nearly  the  composition  of  serpentine ;  it  is  brown 
or  green,  and  has  the  same  metallic  sheen  as  bronzite,  and 
can  also  be  used  as  an  ornamental  stone. 

The  magnesium  of  enstatite  may  be  more  or  less  replaced 
by  other  metals,  such  as  calcium,  manganese,  zinc,  iron 
(ferrous  or  ferric),  aluminium,  sodium,  lithium.  Thus  equal 
molecules  of  magnesia  and  lime  give  diopside,  2SiO2.MgO.CaO 
or  CaMg(SiO3)2,  which  is  chemically  a  combination  of 
enstatite  and  wollastonite,  or  intermediate  between  them. 
Clear  crystals  of  diopside  have  been  cut  as  gem-stones. 
Diopside  is  the  magnesian  silicate  most  easily  reproduced 
artificially.  It  has  been  found  in  masses  resulting  from 
fusion  of  schists  through  fires  in  coal-mines,  and  also  in 
devitrified  masses  of  bottle  glass. 

^Egirite  or  Acmite,  4SiO2.Fe2O3.Na2O  orNaFe'"(SiO3)2, 
is  essentially  a  sodium  and  ferric  meta-silicate,  of  a  dark- 
brown  or  very  dark-green  colour ;  its  hardness  is  6  to  6J, 
and  its  specific  gravity  3*55.  Jadeite  (sometimes  called 
jade)  has  a  similar  composition  to  segirite — 4SiO2.Al2O3.Na2O 
or  NaAl(SiO3)2 — but  with  aluminium  replacing  the  iron, 
and  is  generally  whitish  or  greenish  unless  it  contains  much 
iron.  The  specific  gravity  is  3*30-3*35,  and  it  is  more  easily 
fusible  and  rather  harder  than  nephrite  (true  jade),  and 
colours  flame  yellow.  It  was  used  by  the  ancients  like  flint 
for  making  various  carved  objects,  including  hatchets  and 
other  implements.  It  is  highly  valued  as  an  ornamental 
stone,  especially  by  the  Chinese. 

Spodumene  contains  lithium  instead  of  the  sodium  of 
jadeite,  and  is  usually  light  grey  to  greenish ;  hardness 
nearly  7,  and  specific  gravity  about  3*3  ;  a  rare  emerald- 
green  variety  called  hiddenite  from  North  Carolina  has  been 
brought  into  use  as  a  precious  stone.  Ordinary  spodumene 
is  occasionally  used  as  an  ornamental  stone. 

Augite  (Greek  auge,  "  brightness  ")  is  the  most  important 
of  all  the  pyroxenes.  It  appears  to  be  an  intimate  mixture 


SILICATES  65 

of  two  minerals,  one  corresponding  to  diopside  in  which 
part  of  the  magnesium  has  been  substituted  by  iron,  and  the 
other  corresponding  to  the  rare  mineral  kornerupine, 
SiO2.Al2Og.MgO,  but  with  more  or  less  of  the  alumina  and 
magnesia  replaced  by  ferric  and  ferrous  oxides  respectively. 
The  alumina  and  ferric  oxide  together  amount  to  10  to  15 
per  cent.,  with  also  2  to  6  per  cent,  of  alkalies.  Augite  is 
an  important  constituent  of  volcanic  rocks  and  basalts,  etc. 
It  is  opaque  and  black  or  greenish-black.  The  hardness  is 
5  to  6,  and  the  specific  gravity  3*33  to  3*36. 

THE  AMPHIBOLES. 

The  amphiboles  (from  Greek amphibolos,  "ambiguous," 
because  some  of  the  minerals  had  been  confounded  with 
tourmaline),  as  previously  mentioned,  closely  resemble  the 
pyroxenes  in  their  chemical  composition  and  general 
characters,  but  differ  in  the  inclination  of  their  cleavage 
planes,  the  angles  being  124°  and  56°,  whilst  for  pyroxenes 
they  are  93°  and  87°.  These  cleavage  planes  are  indicated 
by  cracks  or  striae  when  minerals  or  rocks  are  examined  with 
a  lens,  or  in  thin  sections  under  the  microscope.  The 
amphiboles  have  lower  specific  gravities  than  the  corre- 
sponding pyroxenes.  As  in  the  case  of  the  pyroxenes,  the 
magnesian  silicates  form  isomorphous  mixtures  and  mixed 
crystals  of  very  variable  composition,  but  anthophyllite  is 
essentially  magnesium  and  iron  silicate,  SiO2»(Mg,Fe)O,  or 
(Mg,Fe)SiO3,  and  occurs  as  brownish  fibrous  or  lamellar 
masses  in  mica  schists. 

Tremolite  is  a  magnesium  silicate  in  which  one-fourth 
of  the  magnesium  is  replaced  by  calcium,  the  formula  being 
4SiO2-3MgO.CaO  or  Mg3Ca(SiO3)4.  It  is  a  variety  of  amphi- 
bole  occurring  in  long,  flat  crystalline  fibres,  or  in  coarse 
columnar  forms,  and  as  it  contains  little  or  no  iron  the  colour 
is  white,  grey,  pale  greenish  or  yellowish.  The  hardness  is 
about  5j,  and  the  specific  gravity  2 '9  (the  lowest  for  an 
amphibole) .  Actinolite  has  the  same  composition  as  tremo- 
lite,  except  that  part  of  the  magnesium  of  the  latter  is 
c.  5 


66  SILICA   AND  THE  SILICATES 

replaced  by  iron.  It  forms  radiating  groups  of  acicular 
crystals,  hence  the  name  (Greek,  actis,  "a.  ray,"  and  lithos, 
"  a  stone  ")  ;  the  colour  is  bright  green  or  greyish  green.  The 
most  important  amphibole  industrially  is  asbestos  (a  Greek 
word,  meaning  "unconsumable"),  which  is  a  pale — white, 
green,  or  brownish — fibrous  variety  of  tremolite  or  actinolite. 
The  fibres  are  sometimes  so  fine  and  flexible  that  they  can 
be  woven  into  cloth.  Amianthus,  a  corruption  of  amiantus 
(Greek,  amiantos,  "  undefiled  "),  is  a  term  applied  to  the  finer 
kinds  of  asbestos.  Cloth  made  of  this  material  was  used  in 
the  process  of  cremation  so  that  the  ashes  of  the  body  might 
be  kept  separate  from  the  fuel  ashes.  Incombustible  wicks 
were  also  made  of  it.  Napkins  of  asbestos  were  made  by 
the  ancients,  and  cleansed  (when  soiled)  by  being  thrown 
into  the  fire.  Other  uses  are  in  gas  stoves,  for  lining  iron 
safes,  as  a  packing  for  steam  pipes  and  boilers,  and  in 
Corsica  asbestos  has  been  mixed  with  clay  to  make  a  kind 
of  pottery.  Short-fibred  asbestos  is  utilized  for  making 
paper  and  cardboard,  etc.,  for  heat-insulating  purposes,  etc. 
Chrysotile  (fibrous  serpentine)  is  often  used  as  a  substitute 
for  asbestos.  The  South  African  blue  asbestos  or  Cape 
asbestos  is  crocidolite. 

Jade  or  Nephrite,  4SiO2.CaO.3MgO  or  CaMg3(SiO3)4, 
is  essentially  a  silicate  of  magnesium  and  calcium,  often 
regarded  as  a  compact  variety  of  tremolite  or  actinolite, 
according  as  it  is  whitish  or  greenish.  It  is  very  tough,  has 
a  hardness  of  about  6J,  and  specific  gravity  about  3.  It  is 
used  like  jadeite  for  ornaments,  etc.  The  colour  is  mostly 
green,  grey,  or  white.  L,ike  jadeite,  it  has  always  been  highly 
prized  by  the  Chinese,  and  also  by  the  Maoris  in  New  Zealand. 
Jade  implements  have  been  found  widely  distributed  in 
Alaska  and  British  Columbia.  In  some  cases  nephrite  has 
been  formed  by  alteration  of  jadeite. 

Crocidolite  is  a  combination  of  sodium  ferric  metasilicate 
and  ferrous  metasilicate,  NaFe'"(SiO3)2.Fe  SiO3,  and  is  a 
blue  fibrous  amphibole  mineral  resembling  asbestos  ;  it  is 
sometimes  called  blue  asbestos  and  Cape  asbestos  (in  contra- 
distinction to  the  Canadian  chrysotile  or  "  white  asbestos  "). 


SILICATES  67 

It  also  occurs  compact.  It  is  worked  industrially  in  South 
Africa,  where  it  is  used  like  asbestos.  It  undergoes  altera- 
tion by  removal  of  alkali  and  conversion  of  ferrous  iron 
into  ferric  condition,  accompanied  by  deposition  of  silica 
between  the  fibres  or  by  their  replacement  by  silica.  The 
hard  silicious  mineral  thus  formed  is  a  blue  or  golden-brown 
ornamental  stone  called  "  hawk's-eye  "  or  "  tiger-eye  " 
respectively.  The  latter  represents  the  final  alteration 
product,  and  is  often  called  crocidolite  though  it  is  practically 
only  a  mixture  of  quartz  with  brown  oxide  of  iron.  When 
polished,  hawk's-eye  or  tiger-eye  has  a  silky  lustre.  Bach 
contains  about  93  per  cent,  silica,  most  of  the  remainder 
being  iron  oxide  (partly  ferrous  in  the  case  of  hawk's-eye). 

Hornblende  occupies  the  same  position  among  the 
amphiboles  as  augite  among  the  pyroxenes,  and  forms  a 
common  mineral  component  of  many  igneous  and  meta- 
morphic  rocks,  especially  those  whose  constituents  have 
assumed  entirely  or  mainly  a  crystalline  structure.  In  most 
of  its  physical  characters  it  closely  resembles  augite,  but  in 
common  with  the  other  amphiboles  it  differs  as  regards 
cleavage  angles  and  certain  optical  properties.  Hornblende 
is  also  generally  similar  to  augite  in  chemical  composition, 
sometimes  containing  over  20  per  cent,  of  alumina  and 
ferric  oxide. 

HYDRATED  MAGNESIUM  SHJCATES. 

These  are  very  abundant  in  nature,  but  only  in  crypto- 
crystalline  masses — never  well-formed  crystals.  Hydrated 
silicates  are  not  known  corresponding  to  the  two  simple 
anhydrous  magnesian  silicates.  The  most  important  of  the 
natural  hydrated  silicates  of  magnesia  are  talc,  steatite, 
meerschaum,  and  serpentine. 

Talc,  4$iO2.3MgO.H2O  or  H2Mg3Si4O12,  is  a  soft, 
friable  material,  unctuous  to  the  touch,  and  yields  to  the 
finger  nail.  It  occurs  sometimes  in  thin  hexagonal  plates, 
splitting  into  lamellae.  The  colour  is  white,  grey,  green, 
brown,  or  red,  the  hardness  i  to  ij,  and  specific  gravity 


68  SILICA   AND  THE  SILICATES 

about  27.  Common  talc  coloured  by  a  vegetable  extract 
forms  the  rouge  used  by  ladies  as  a  cosmetic.  Talc  powder 
is  used  to  make  new  boots  and  gloves  slip  on  more  easily, 
and  in  like  manner  it  is  used  for  reducing  friction  in  machinery. 
Talc  is  also  used  by  tailors  as  a  kind  of  chalk  for  making 
lines  on  cloth.  It  was  formerly  used  in  porcelain  manu- 
facture to  increase  the  translucency.  Other  uses  for  talc 
are,  for  the  removal  of  grease  from  cloth  and  silk,  as  a  soft 
polishing  powder,  as  a  coating  for  pills,  in  the  manufacture  of 
paper,  and  for  adulterating  soap. 

Steatite  or  Soapstone  is  a  massive  variety  of  talc 
without  foliated  structure.  It  is  easily  cut,  and  when  heated 
becomes  very  hard.  It  is  worked  into  ornaments  by  the 
Chinese,  and  is  made  into  gas  burners,  linings  for  stoves, 
electric  insulators,  and  is  also  employed  for  other  purposes, 
including  most  of  those  for  which  ordinary  talc  is  used  (see 
above). 

Agalite  is  a  trade  name  for  a  fibrous  variety  of  talc. 
It  is  greenish- white,  and  is  readily  reduced  to  short  fine 
fibres.  It  is  nearly  all  used  in  the  American  paper  trade, 
giving  weight  and  body,  and  producing  a  fine  gloss  on  the 
surface  of  the  paper. 

Meerschaum  (meaning  sea  foam),  3SiO2.2MgO.2H2O, 
is  a  soft  white  mineral  sometimes  found  floating  on  the 
Black  Sea.  It  has  also  been  called  sepiolite  from  a  rather 
fanciful  resemblance  to  the  "  bone  "  of  the  cuttle-fish  (sepia). 
It  is  sometimes  grey  or  creamy  in  tint,  has  a  hardness  of 
about  2  (being  easily  scratched  by  the  finger-nail),  and  a 
specific  gravity  of  about  i  to  1-28.  Most  of  the  commercial 
meerschaum  comes  from  Asia  Minor,  where  it  is  associated 
with  magnesium  carbonate  (magnesite)  ;  both  these  minerals 
have  resulted  from  the  decomposition  of  serpentine. 
Meerschaum  also  occurs  to  a  smaller  extent  in  a  number  of 
other  localities.  It  has  sometimes  been  used  instead  of 
soap  and  fuller's  earth,  but  its  main  use  is  for  making 
tobacco-pipes  and  cigar-holders ;  these  are  first  roughly 
shaped  by  scraping,  and  then  polished  with  wax ;  the 
rudely -shaped  articles  are  next  sent  to  Vienna,  etc.,  and  are 


SILICATES  69 

turned  and  carved,  smoothed  with  glass-paper  and  Dutch 
rushes,  heated  in  wax  or  stearine,  and  finally  polished  by 
means  of  bone-ash,  etc. 

In  preparing  substitutes  for  meerschaum,  shavings, 
dust,  chips,  etc.,  of  real  meerschaum  are  worked  up  with 
freshly  precipitated  silicates  of  aluminium,  calcium,  magne- 
sium, etc.,  as  binders,  three  grades  of  ware  being  sorted  out. 
Other  materials  used  include  celluloid,  infusorial  earth, 
casein,  water-glass,  zinc  oxide,  phosphates,  glue,  calcium 
sulphate,  caustic  potash,  etc. 

Serpentine  is  the  most  abundant  of  the  hydrated 
silicates  of  magnesia,  being  mainly  made  up  of  a  silicate 
having  the  composition  2$iO2.3MgO.2H2O.  Chrysotile 
is  a  fibrous  variety  resembling  asbestos,  and  is  often 
used  in  the  same  way  as  asbestos.  Serpentine  is  mostly 
green,  but  generally  shows  several  tints,  arranged  in  dots 
and  streaks,  etc.  The  name  serpentine  is  derived  from 
a  supposed  resemblance  to  the  markings  on  a  serpent's 
skin.  The  hardness  is  3  to  4,  and  specific  gravity  about  2*5 
The  purer  translucent  and  massive  serpentines  are  called 
precious  or  noble  serpentine.  Though  a  soft  mineral  (and 
easy  to  work)  serpentine  takes  a  good  polish,  and  accord- 
ingly is  in  demand  as  an  ornamental  stone  for  making 
chimney-pieces,  columns,  table  tops,  vases,  etc.  ;  many 
such  ornaments  have  been  made  in  Cornwall,  the  I/izard 
district  being  famous  for  its  serpentine  deposits.  Formerly 
much  serpentine  was  sent  to  Bristol  to  be  made  into  magne- 
sium carbonate. 

The  hard  compact  variety  of  serpentine  called  bowenite 
(specific  gravity  2*6)  and  other  similar  minerals  are  readily 
distinguished  by  their  specific  gravity  from  jadeite  and 
nephrite  (jade),  the  specific  gravity  of  these  latter  being 
about  3-0  to  3*3. 

The  hydrated  magnesium  silicates  are  all  derived  from 
pyroxenes  and  olivine  rocks,  the  alteration  being  produced 
by  the  combined  action  of  water  and  carbonic  acid  (carbon 
dioxide),  as  is  indicated  by  the  magnesium  carbonate  and 
calcareous  materials  with  which  they  are  often  associated. 


70  SILICA   AND   THE  SILICATES 

ANHYDROUS  SIUCATES  OF  ALUMINA. 

Only  one  anhydrous  silicate  of  alumina  of  very  definite 
composition  is  known  to  exist,  and  it  corresponds  to  the 
formula  SiO2.Al2O3  or  Al2SiO6.  This  occurs  in  nature  as 
three  (if  not  four)  different  minerals  with  different  crystalline 
forms,  andalusite,  cyanite,  or  disthene,  and  sillimanite  or 
fibrolite.  Andalusite  crystallizes  in  the  orthorhombic 
system.  Its  specific  gravity  is  3*15,  and  hardness  7^.  The 
colour  is  grey  to  brownish  in  opaque  crystals,  green  or  reddish- 
brown  in  transparent  pebbles,  and  it  is  harder  and  more 
infusible  than  felspar.  The  pebbles  make  good  gem-stones. 
Andalusite  occurs  chiefly  in  gneiss,  mica-slate,  and  clay- 
slate. 

Chiastolite  is  a  variety  of  andalusite  occurring  in  white 
or  grey  rhombic  prisms,  which  enclose  some  of  the  slate  or 
argillaceous  schist  in  which  the  crystals  are  embedded,  and 
the  dark-coloured  carbonaceous  matter  therein  is  arranged 
so  that  a  cross  section  of  a  crystal  shows  a  dark  cross  (at 
the  middle)  or  a  rectangle  (at  the  ends)  or  some  intermediate 
figure.  Cut  and  polished  crystals  of  chiastolite  are  worn  as 
charms. 

Cyanite,  Kyanite,  or  Disthene  crystallizes  in  the 
triclinic  system.  Its  specific  gravity  is  about  3*6.  The 
colour  is  generally  pale  blue  (whence  the  name  cyanite  or 
kyanite),  but  sometimes  white,  grey,  greenish,  yellowish, 
and  black.  The  hardness  varies,  being  5  on  the  lateral  planes 
and  7  on  the  extremities  (whence  the  name  disthene).  It 
occurs  chiefly  in  gneiss  and  mica-slate.  Transparent  blue 
specimens  are  sometimes  used  as  gem-stones. 

Sillimanite  or  Fibrolite  crystallizes  in  the  orthorhombic 
system,  but  in  forms  different  from  those  of  andalusite, 
which  also  differs  in  some  of  its  optical  properties.  The 
specific  gravity  of  sillimanite  is  almost  3-2,  its  hardness 
about  7,  and  its  melting  point  1850°  C.  The  colour  is  greyish, 
greenish,  or  brown.  The  name  fibrolite  is  sometimes 
restricted  to  a  fibrous  massive  variety,  consisting  of  a 
confused  mass  of  very  fine  crystalline  needles ;  this  was 


SILICATES  71 

used  for  making  many  tools  in  the  Stone  age.  Sillimanite 
occurs  in  gneiss  and  schists.  Sillimanite  is  produced  by 
the  action  of  heat  (at  about  1350°  C.)  on  andalusite  or 
cyanite,  or  even  on  kaolinite  or  kaolin,  or  other  hydrated 
compounds  of  alumina  and  silica,  and  its  formation  is 
attended  by  a  remarkable  disengagement  of  heat.  Vernadsky 
used  this  heat  in  producing  Sillimanite  by  fusion  of  its 
components  (alumina  and  silica)  on  heating  to  about  1600°  C. 
Iye  Chatelier  gives  the  following  approximate  fusing  points : — 

Al2O-j-SiO2        1850°  C.      Al2O3+i5SiO2    1600°  C. 
Al2O3+2SiO2     1770°          Al2O3-f20$iO2     1670' 
Al2O3+ioSiO2  1630°  SiO2     1770' 

A12O3  2000' 


Ceramic  products  burned  to  high  temperatures,  especially 
refractory  clay  firebricks  and  hard  porcelain  contain  long 
needles  of  sillimanite.  Its  hardness  readily  distinguishes 
sillimanite  from  anthophyllite  (one  of  the  amphiboles)  for 
which  it  was  formerly  mistaken. 

Topaz,  SiO2.Al2O2F2  or  Al2SiO4F2,  differs  in  composition 
from  the  three  last  minerals  in  having  fluorine  substituted 
for  a  portion  of  their  oxygen.  When  topaz  is  heated  very 
strongly,  a  mixture  of  sillimanite  and  alumina  is  produced. 
Topaz  crystallizes  in  the  rhombic  system.  The  specific 
gravity  is  about  3*5,  and  the  hardness  is  8.  Topaz  is  typi- 
cally yellow,  but  also  occurs  colourless,  blue,  green,  and  pink. 
Most  varieties  of  topaz  are  cut  for  gems.  Topaz  becomes 
electrified  by  moderate  heating  or  friction,  which  distin- 
guishes the  colourless  variety  from  the  diamond,  and  topaz 
in  general  from  many  other  minerals ;  the  same  treatment 
causes  .  yellow  varieties  to  become  pink.  Aquamarine 
(beryl)  and  chrysolite  (olivine)  are  sometimes  substituted 
for  topaz,  but  are  easily  distinguished  by  differences  in 
hardness,  etc.  Yellow  quartz  is  sometimes  called  Spanish 
topaz,  Scotch  topaz,  or  occidental  topaz  ;  oriental  topaz  is 
yellow  corundum  (consisting  almost  exclusively  of  alumina). 
The  colourless  topaz  exhibits  far  more  lustre  than  rock 
crystal  (quartz),  and  the  purest  specimens,  when  cut  in 


72  SILICA   AND  THE  SILICATES 

facets  like  the   diamond,   rival  the   latter   in   lustre   and 
brilliance. 

Staurolite  or  Staurotide  is  another  mineral  conveniently 
placed  here,  though  its  composition  is  very  variable  ;  it 
contains,  besides  silica  and  alumina,  sometimes  a  considerable 
amount  of  iron  oxide,  and  nearly  always  1*5  per  cent,  of 
water  which  can  only  be  driven  off  at  a  red  heat.  Staurolite 
crystallizes  in  the  rhombic  system.  Twin  crystals  in  the 
form  of  a  cross  are  common,  whence  the  name  (meaning  cross 
stone,  from  Greek  stauros,  "  a  cross  ") .  The  specific  gravity  is 
about  3 -6  and  the  hardness  about  7.  The  colour  is  generally 
reddish-brown  to  brownish-black. 


HYDRATED  COMPOUNDS  OF  SIUCA  AND  ALUMINA, 

These  are  often  grouped  as  salts  of  certain  silicic  acids — 
H4SiO4,  H6Si2O7,  H2SiO3,  H2Si2O5,  and  H4Si3O8.  The 
present  tendency  is  to  consider  them  as  alumino-silicic  acids, 
as  proposed  by  Vernadsky,  whose  hypothesis  takes  account 
of  the  important  fact  that  the  same  proportions  of  silica 
and  alumina  are  present  in  a  number  of  silicates  containing 
also  oxides  of  other  metals.  Five  alumino-silicic  acids  were 
enumerated,  but  Dr.  J.  W.  Mellor  recognizes  six,  as  follows  : — 


Aluminomonosilicic  acid  (Allophanic  type) 
Alumino-disilicic  acid  (Kaolinic  type) 
Alumino-trisilicic  acid  (Natrolitic  type)    . . 
Alumino-tetrasilicic  acid  (Pyrophyllitic  type) 
Alumino-pentasilicic  acid  (Chabazitic  type) 
Alumino-hexasilicic  acid  (Felspathic  type) 


Al2O3.SiO2.wH2O 
Al2O3.2SiO2.wH2O 
Al2O3.3SiO2.wH2O 
Al2O3.4SiO2.wH2O 
Al2O3.5SiO2.wH2O 
Al2O3.6SiO2.wH2O 


The  value  of  n  in  the  above  formulae  in  the  case  of 
several  of  the  types  may  be  one,  two,  or  three  ;  in  other 
cases  it  does  not  assume  all  these  values.  In  a  few  instances 
n  equals  four  or  five. 

Hydrated  compounds  resembling  kaolinite  behave  like 
acids,  as  by  displacing  carbon  dioxide  from  carbonates, 
sulphur  trioxide  from  sulphates,  chlorine  from  chlorides, 
etc.  Kaolinite  and  clays  are  therefore  considered  to  be 
more  or  less  impure  alumino-silicic  acids  rather  than 


SILICATES  73 

hydrated  aluminium  silicates.  Zeolites  and  related  com- 
pounds are  regarded  as  salts  of  alumino-silicic  acids. 

As  there  is  no  known  means  of  determining  the  exact 
molecular  weight  of  such  compounds,  the  ordinary  molecular 
proportion  of  alumina  may  be  taken  as  the  unit,  but  to 
avoid  fractions  of  atomic  weights  of  certain  bodies,  the 
formulae  so  obtained  may  be  multiplied  so  as  to  give  whole 
numbers.  Possibly  it  might  be  better  to  base  the  formulae 
on  six  molecules  of  silica  instead  of  one  molecule  of  alumina, 
but  in  any  case  it  remains  hypothetical,  and  so  it  is  of  little 
importance  which  of  these  alternatives  may  be  adopted. 

Allophane,  SiO2.Al2O3.5H2O  (or  perhaps  4H2O),  has  a 
specific  gravity  of  about  1-9,  and  hardness  about  3.  There 
is  no  distinct  crystalline  structure.  It  is  the  only  compound 
containing  both  silica  and  alumina  which  is  very  easily 
attacked  by  dilute  acids  with  separation  of  gelatinous  silica. 
When  impregnated  with  water,  allophane  becomes  trans- 
lucent, sometimes  almost  transparent.  It  occurs  of  various 
colours.  A  snow-white  allophane  finds  industrial  use  in 
France  by  treatment  with  sulphuric  acid  in  order  to  obtain 
aluminium  sulphate  free  from  iron.  Allophane  and  certain 
other  hydrated  alumina  silicates  are  sometimes  considered 
to  be  mixtures  of  colloidal  alumina  and  silica.  Sillimanite 
may  be  the  anhydride  corresponding  to  allophane. 

Pyrophyllite,  4SiO2.Al2O3.H2O,  or  H2Al2Si4Oi2,  is  an 
alumino-tetrasilicic  acid.  Its  specific  gravity  is  about  2*9, 
and  its  hardness  is  1-2.  It  occurs  in  lamellae,  and  also 
massive,  like  talc,  etc.,  which  it  greatly  resembles.  The 
colour  is  generally  white  or  pale  green.  Several  minerals 
contain  the  same  proportions  of  silica  and  alumina,  but 
different  proportions  of  water.  They  are  to  be  regarded  as 
varieties  of  alumino-tetrasilicic  acid.  Pyrophyllite  can  be 
considered  as  the  acid  corresponding  to  leucite,  jadeite, 
analcite,  etc.  Pyrophyllite  is  applied  to  many  of  the  purposes 
for  which  talc  is  used.  The  compact  variety  of  pyrophyllite 
is  made  to  serve  for  slate  pencils  and  tailors'  chalk,  and  also 
for  Chinese  carvings.  The  name  is  derived  from  Greek  pur, 
"fire,"  and  phullon,  "a  leaf/'  in  allusion  to  the  effect  of 


74  SILICA   AND  THE  SILICATES 

heat  in  causing  separation  of  the  laminae  in  foliated 
varieties. 

The  Japanese  mineral  roseki,  resembling  pyrophyllite,  is 
crushed,  washed,  and  prepared  much  in  the  same  way  as 
China  clay,  and  the  product  used  for  making  firebricks, 
and  to  a  smaller  extent  in  making  porcelain  and  paper. 

Agalmatolite  or  Pagodite  is  a  soft  hydrated  compound 
of  silica  with  certain  oxides,  which  the  Chinese  carve  into 
images  and  ornaments.  It  really  includes  three  different 
minerals :  pyrophyllite,  steatite,  and  pinite,  the  last 
named  being  an  alteration  product  consisting  largely  of  a 
finely  scaly  muscovite. 

Fuller's  Earth  is  essentially  a  hydrated  compound  of 
silica  and  alumina  but  the  latter  is  often  largely  replaced  by 
ferric  oxide.  It  is  soft  and  opaque,  and  has  a  greasy  feel. 
The  colour  is  generally  greenish-brown,  or  greenish-grey,  but 
sometimes  blue.  When  placed  in  water,  it  becomes  trans- 
lucent, and  falls  into  a  pulpy  powder  without  forming  a 
paste  as  clay  does.  Fuller's  earth  was  formerly  largely  used 
by  cloth  manufacturers  for  "  fulling  "  woollen  cloth,  that 
is,  cleansing  it  by  absorbing  oil  and  grease.  I,atterly  it  has 
been  to  a  great  extent  replaced  by  other  substances  for  that 
purpose.  Fuller's  earth  occurs  at  Nutfield,  near  Reigate, 
Surrey,  and  other  places  in  the  south  of  England  and  in 
Scotland. 

Fuller's  earth  is  sometimes  simply  dried,  but  in  other 
cases  it  is  afterwards  ground.  In  some  cases  it  is  made  into 
a  slurry  with  water,  which  carries  off  the  finer  material  to 
settle  in  tanks,  to  be  afterwards  dried.  It  is  now  very 
much  used  in  the  filtration  of  mineral  oils,  and  for  removing 
the  colour  from  certain  vegetable  oils,  greases,  and  fats. 
It  is  also  used  for  making  certain  soaps  and  cleaning  pre- 
parations, for  pigments,  and  for  printing  wallpapers. 

Kaolinite,  2SiO2.Al2O3.2H2O,  and  halloysite,  with  3H2O 
or  4H2O  instead  of  2H2O,  may  be  regarded  as  alumino- 
disilicic  acids,  and  others  are  rectorite  with  iH2O  and  newtonite 
with  5H2O,  the  two  latter  being  of  rare  occurrence.  Halloy- 
site is  soft,  and  adheres  to  the  tongue ;  it  has  a  specific 


SILICATES  75 

gravity  of  2*0  to  2 '2,  and  hardness  i  to  2.  The  colour  of 
halloysite  is  generally  white,  with  a  slight  bluish  tinge, 
but  may  be  grey,  etc.  When  strongly  heated  it  becomes 
much  harder,  and  of  a  milk-white  colour.  According  to 
J.  W.  Mellor  the  molecule  of  halloysite  seems  to  break 
down  below  200°  C.  It  has  not  the  plasticity  of  clay. 
Kaolin  and  clay  consist  mainly  of  kaolinite  (which  occurs 
very  rarely  in  a  pure  state),  and  have  the  property  of 
forming  with  water  plastic  pastes.  Kaolin  has  been  observed 
in  some  cases  to  be  the  decomposition  product  of  granite 
and  other  rocks  containing  felspar. 

Kaolin,  or  China  clay,  is  generally  very  pure,  and  includes 
little  or  no  iron  in  its  composition,  and  very  small  amounts 
of  potash  and  soda ;  it  is  used  in  the  manufacture  of 
porcelain  and  pottery,  and  for  making  aluminium  sulphate. 
Its  usual  impurities  comprise  only  quartz  sand,  felspar, 
and  mica,  all  derived  from  the  parent  rock.  The  so-called 
rational  analysis  of  kaolin  (or  of  clay)  is  designed  to  ascertain 
the  proportions  of  these  substances  mixed  with  the  clay 
basis  of  the  material.  The  method  depends  on  the  assump- 
tion that  only  the  clay  substance  is  attacked  by  sulphuric 
acid,  and  that  the  silica  liberated  from  the  clay  can  then  be 
dissolved  by  a  solution  of  soda.  It  has,  however,  been  shown 
by  J.  W.  Mellor  that  the  assumption  is  erroneous, 
and  better  results  can  usually  be  obtained  by  calculation 
from  the  ultimate  chemical  analysis.  Thus  the  alkalies 
(potash  and  soda)  present  are  first  calculated  to  felspar 
(6SiO2.Al2O3.K2O,  and  the  corresponding  sodium  compound), 
the  alumina  not  required  for  this  is  calculated  as  kaolinite 
(Al2O3.2SiO2.2H2O),  and  the  remaining  silica — not  needed 
for  either  felspar  or  kaolin — is  reckoned  as  quartz.  Certain 
refinements  may  be  introduced  if  desired,  as  calculating 
mica  instead  of  felspar,  etc. 

Kaolin  usually  has  a  fine  white  colour,  which  it  keeps 
after  heating.  Inferior  qualities  burn  to  a  j^ellowish  colour, 
and  some  have  a  similar  tint  in  the  raw  state.  Unlike  most 
clays,  kaolin  is  always  practically  free  from  titanium,  and  it 
has  generally  less  than  i  per  cent,  of  iron.  Most  of  the 


76  SILICA   AND  THE  SILICATES 

impurities  can  be  removed  by  washing  with  water.  In 
ordinary  washing  by  water,  the  particles  of  kaolin  become 
coagulated  through  the  presence  of  calcium  salts,  and  form 
small  clots  which  are  soon  deposited. 

Clay  is  often  coloured  grey  or  black  by  the  presence  of 
more  or  less  bituminous  organic  matters,  which  of  course 
burn  off  when  the  clay  is  heated.  The  general  presence  in 
clays  of  i  to  2  per  cent,  of  titanium  dioxide  (often  called 
titanic  acid)  produces  on  heating  a  yellowish  grey  colora- 
tion. The  titanium  greatly  exaggerates  the  red  coloration 
due  to  the  presence  of  iron.  Kaolins  containing  i  per  cent. 
of  iron  oxide  burn  nearly  white,  whereas  clays  with  the 
same  content  in  iron  burn  to  rose-colour  and  nearly  red. 
With  a  few  hundredths  of  iron  the  coloration  becomes 
blood-red. 

Iron  colours  some  clays  green,  and  is  then  present  in  the 
state  of  ferric  oxide,  forming  nontronite,  2SiO2.Fe2O3.2H2O, 
which  occurs  in  isomorphous  admixture  with  kaolinite. 

Many  common  clays  are  coloured  reddish-brown  by  the 
ordinary  hydrate  of  ferric  oxide. 

Yellow  ochres  include  iron  in  the  state  of  free  hydrate, 
of  which  the  formula  is  not  known  with  certainty. 

Certain  clays  present  a  blood-red  coloration,  as  also 
does  bauxite  ;  they  contain  free  anhydrous  ferric  oxide. 

Some  clays  of  a  whitish  grey  colour  contain  a  notable 
proportion  of  iron,  but  not  uniformly  distributed ;  the 
colour  due  to  iron  appears  on  burning. 

Clays  always  include  amounts  (sometimes  considerable) 
of  quartz  and  mica,  the  quartz  being  much  the  more  abundant. 
As  they  are  constituents  of  all  clays,  they  can  hardly  be 
regarded  as  impurities. 

Ball  Clay  contains  rather  more  iron  than  kaolin,  and 
appreciably  more  alkali  (potash  and  soda),  and  has  a  cream 
or  ivory  colour  when  burned ;  it  also  contains  more  free 
silica  (quartz)  than  kaolin.  It  is  accordingly  less  refractory 
than  kaolin. 

Fireclay  is  a  refractory  clay  containing  only  a  small 
percentage  of  alkali,  but  enough  oxide  of  iron  (usually 


SILICATES  77 

accompanied  by  titanium)  to  give  it  a  more  pronounced 
colour  after  burning.  Some  fireclays  are  about  as  refractory 
as  kaolin  ;  others  are  less  refractory  than  ball  clay. 

Impure  clays  are  used  for  making  common  building 
materials — bricks  and  tiles.  They  contain  as  impurities 
notable  quantities  of  iron  oxide  and  carbonate  of  lime,  the 
latter  in  particular  rendering  the  material  very  fusible. 
Small  crystals  of  gypsum  (sulphate  of  lime  or  calcium 
sulphate)  also  often  occur,  which  give  rise  to  white  spots  at 
low  temperatures,  and  to  yellow  spots  at  the  temperature  of 
fusion,  the  colour  in  the  latter  case  being  due  to  combined 
oxide  of  iron. 

Calcareous  clays  are  known  as  marls,  but  the  same  term 
is  often  applied  locally  to  fireclays. 

Common  clays  include  impurities  occurring  in  isolated 
fragments,  giving  rise  to  various  inconveniences  after  burn- 
ing. Flint  gives  white  spots  without  disturbing  the  solidity ; 
oxide  of  iron  and  pyrite  produce  holes  bordered  by  a  black 
ring  of  magnetic  oxide,  giving  a  very  disagreeable  appearance 
to  the  brick  without  reducing  the  strength ;  limestone,  in 
grains  larger  than  i  mm.,  leaves  fragments  of  free  lime,  the 
slaking  of  which  makes  the  bricks  or  tiles  split  later. 

Most  of  the  brick  earths  collected  superficially  are  not 
true  clays ;  they  are  complex  mixed  residues  proceeding 
from  disintegration  of  surface  rocks,  in  which  true  clay  is 
present  in  small  quantities  or  sometimes  perhaps  wholly 
absent. 

MONOBASIC  AI<UMINO-DISIIJCATES. 

The  chief  members  of  this  group  are  lime-felspar  and 
nephelite. 

Anorthite  or  Lime-felspar,  2SiO2.Al2O3.CaO,  is  the  most 
basic  of  the  felspars.  It  has  a  hardness  of  6  to  7,  and  a 
specific  gravity  of  about  27.  It  occurs  in  white  or  colourless 
crystals,  and  is  a  constituent  of  many  igneous  rocks,  but  in 
itself  has  no  industrial  importance.  It  is  classed  with  the 
felspar  family  because  it  can  form  mixed  crystals  with 
felspars  in  all  proportions. 


78  SILICA   AND  THE  SILICATES 

Nephelite  or  Nepheline,  2SiO2.Al2O3.(Na,K)2O  or 
(Na,K)Al(SiO4)2.  The  proportion  of  sodium  to  potassium 
is  about  3Na  :  K.  In  volcanic  rocks,  nephelite  and  leucite 
take  the  place  of  the  felspars  present  in  older  rocks.  Nephe- 
lite has  a  hardness  of  5|  to  6,  and  specific  gravity  about  2-5. 
Two  varieties  of  nephelite  are  met  with.  The  glassy 
nephelite  occurs  in  small  colourless  and  transparent  crystals 
and  grains. 

The  other  variety,  el&olite,  occurs  either  as  large  rough 
crystals  or  (more  frequently)  as  irregular  masses,  which  are 
opaque  or  at  best  translucent,  and  of  a  reddish,  greenish, 
brownish,  or  grey  colour.  When  of  a  good  green  or  red  tint 
and  showing  a  kind  of  "  cat's-eye "  effect,  elseolite  is 
occasionally  cut  with  a  convex  surface  as  a  gem-stone. 
The  closely  related  mineral  cancrinite  is  also  sometimes  cut 
as  a  gem-stone  when  bright  yellow ;  this  is  a  hydrated 
mineral  of  the  nephelite  type  as  regards  composition,  but 
with  calcium  instead  of  potassium,  and  including  some 
sodium  carbonate ;  its  hardness  is  5  \  to  6,  its  specific  gravity 
about  2*5,  and  its  colour  white,  grey,  yellow,  green,  blue,  or 
light  reddish.  Two  other  related  minerals  are  socialite, 
which  includes  some  chloride,  and  hauyne  (or  hauynite), 
which  includes  some  sulphate  in  its  composition,  both  being 
frequently  of  a  blue  colour.  Hauyne  has  sometimes  been 
used  as  a  gem-stone,  and  sodalite  has  been  quarried  in 
Canada  as  an  ornamental  stone  under  the  trade  names 
"  Princess  Blue  "  and  "  Alomite  "  ;  it  takes  a  good  polish, 
and  looks  somewhat  like  marble,  but  is  harder  (its  hardness 
being  5j)  ;  its  specific  gravity  is  2-3. 

Lapis  Lazuli,  Azure  Stone,  or  Azure  Spar,  sometimes 
called  lazulite,  is  a  valuable  ornamental  stone  because  of 
its  fine  blue  colour.  It  was  also  prepared  by  grinding  and 
levigation  to  give  the  pigment  ultramarine,  but  for  that 
purpose  is  now  practically  superseded  by  artificial  ultra- 
marine. 

L,apis  lazuli  generally  occurs  in  masses  with  a  finely 
granular  structure,  and  only  rarely  as  distinct  crystals. 
Its  hardness  is  about  5j,  and  specific  gravity  about  4*4. 


SILICATES  79 

Some  varieties  are  green,  violet,  red,  or  colourless.  Generally 
lapis  lazuli  is  opaque,  with  only  slight  translucency  where 
the  edges  are  thin.  Its  composition  is  variable,  and  micro- 
scopic investigation  has  shown  that  it  is  a  rock  rather  than  a 
mineral. 

Most  lapis  lazuli  consists  of  a  blue  mineral  allied  to 
sodalite  and  hauyne,  called  lazurite,  which  is  a  sodium  and 
aluminium  silicate  containing  sulphur,  of  the  formula 
Na4(NaS3Al)Al2(SiO4)3.  Mixed  with  the  lazurite  is  a 
varying  proportion  of  hauyne,  and  along  with  the  blue 
hauyne  and  lazurite  are  the  pyroxene  diopside  (without 
iron),  an  amphibole  (koksharovite),  orthoclase  and  other 
felspars,  a  mica  resembling  muscovite,  apatite  (calcium 
phosphate  with  some  fluoride  and  chloride),  zircon,  sphene 
(titanite),  calcite,and  pyrite,  the  latter  being  often  altered 
to  limonite.  I,apis  lazuli  generally  occurs  in  crystalline 
limestone. 

Ultramarine  was  first  made  artificially  on  a  large  scale 
by  Guimet  in  1828,  but  was  independently  prepared  on  a 
small  scale  by  Gmelin  about  the  same  time.  It  is  also  said 
to  have  been  made  by  Kottig,  at  Meissen. 

Artificial  Ultramarine  Blue  is  sold  as  a  bright  azure  blue 
impalpable  powder,  which  is  insoluble  in  water,  and  contains 
the  elements  sodium,  aluminium,  silicon,  sulphur,  and  oxygen. 
When  heated  with  water  in  a  sealed  tube  to  200°-300°,  a 
colourless  residue  free  from  sulphur  is  left,  and  sodium 
sulphide  is  found  in  the  solution. 

Ultramarine  is  not  changed  by  alkalies,  but  dilute  acids 
readily  decompose  it,  precipitating  sulphur,  and  liberating 
sulphuretted  hydrogen.  The  presence  of  a  polysulphide 
seems  to  be  indicated  by  these  reactions,  but  ultramarine 
is  not  affected  by  concentrated  and  fuming  sulphuric  acid  or 
certain  other  reagents  which  promptly  decompose  poly- 
sulphides  and  thiosulphates.  Heating  to  redness  causes 
ultramarine  to  lose  its  brilliancy  and  to  become  greenish, 
but  there  is  no  other  noteworthy  change ;  the  blue  colour 
therefore  can  hardly  be  attributed  to  free  sulphur.  When 
heated  strongly  in  hydrogen,  ultramarine  blue  becomes 


8o  SILICA   AND  THE  SILICATES 

brownish  yellow,  and  when  heated  to  dull  redness  in  chlorine 
it  becomes  violet.  It  possesses  hydraulic  properties,  and 
so  increases  the  binding  power  of  cement. 

By  substitution  of  silver  for  sodium,  a  Silver  Ultramarine 
of  dark  yellow  colour  is  obtained,  but  no  silver  sulphide. 
By  fusing  this  substance  with  metallic  chlorides  or  iodides, 
the  silver  is  replaced  by  another  metal,  potassium  or  lithium 
giving  a  blue  ultramarine,  barium  yellowish-brown,  zinc 
violet,  etc. 

Ultramarine  blue  crystallizes  in  the  regular  or  cubic 
system,  isomorphous  with  sodalite  and  hauyne. 

Green  Ultramarine  is  formed  as  an  intermediate  product 
in  the  manufacture  of  ultramarine  blue,  being  converted 
into  the  latter  by  roasting  with  sulphur. 

White  Ultramarine  is  obtained  when  air  is  excluded 
during  the  roasting  of  the  materials  for  making  ultramarine  ; 
it  is  converted  into  blue  ultramarine  by  heating  in  either 
oxygen,  sulphur  dioxide,  or  chlorine. 

Red  Ultramarine  was  obtained  by  Scheffer  by  heating 
the  ordinary  raw  materials  strongly  in  a  muffle  with  free 
exposure  to  air ;  it  contains  more  aluminium  and  less  sodium 
than  the  blue  variety.  Red  ultramarine  has  also  been 
prepared  by  passing  chlorine  and  steam  over  heated  ultra- 
marine blue. 

Yellow  Ultramarine  is  formed  by  heating  the  red  variety 
in  air  at  360°  for  a  short  time. 

Violet  Ultramarine  is  obtained  by  first  passing  chlorine 
over  ultramarine  at  410°,  before  it  has  acquired  its  blue 
colour,  when  a  green,  and  then  a  reddish  yellow  product  is 
obtained,  and  then  heating  the  latter  with  sodium  hydroxide. 
It  has  also  been  prepared  in  several  other  ways.  It  has  been 
manufactured  from  ultramarine  blue  by  heating  thin  layers 
(on  earthenware  trays)  at  280°,  passing  steam  over  it  to 
remove  sulphur,  then  lowering  the  temperature  to  160°, 
and  leading  a  mixture  of  steam  and  chlorine  over  it  for  three 
hours. 

Commercial  blue  ultramarines  are  made  in  three  different 
grades,  containing  different  proportions  of  sulphur  and  silica. 


SILICATES  81 

The  same  proportions  of  raw  materials  and  the  same 
conditions  do  not  give  products  of  identical  composition. 
Thus  in  ultramarine  blue  the  silica  was  found  to  range  from 
38*9  to  427,  the  alumina  from  24*0.  to  29*5,  the  soda  from 
187  to  21*0,  and  the  sulphur  from  I0'8  to  15*4.  In  the 
range  of  different  coloured  products,  the  component  showing 
the  greatest  fluctuations  is  the  sulphur,  and  next  to  that 
the  soda.  No  very  clear  deductions  can  be  drawn  from  the 
variations  in  composition,  for  in  two  similar  blues  the 
proportion  of  sulphur  in  one  was  exactly  double  that  in 
the  6ther,  and  the  only  other  very  marked  difference  was  in 
the  alumina,  that  with  the  lower  sulphur  containing  about 
20  per  cent,  more  alumina  ;  further  the  red,  violet,  and  light 
blue  (in  that  order)  contained  least  soda. 

The  constitution  of  ultramarine  is  very  uncertain. 

Uses  of  Ultramarine. — As  a  body  colour  it  is  used  in 
calico-printing,  and  wallpaper  printing,  for  colouring  writing 
paper  and  printing  ink,  preparing  blue  pigments,  and  blueing 
mottled  soap.  As  a  whitening  agent,  the  great  strength 
and  purity  of  its  blue  tint  correct  the  yellowness  of  writing 
and  printing  papers,  cotton  and  linen  goods,  paper  pulp, 
whitewash,  soap,  starch,  and  even  sugar.  It  is  largely  used 
for  making  square  and  ball  blue  for  laundry  use. 

Manufacture  of  Ultramarine. — Only  blue  and  green 
sodium — sulphur — ultramarines  are  made  on  a  large  scale. 
The  three  chief  commercial  varieties  of  ultramarine  blue 
are : — 

Glauber  Salt  or  Sulphate  Ultramarine,  which  is  the 
palest  variety,  and  has  a  slight  greenish  tinge.  It  has  small 
covering  power,  and  is  more  readily  attacked  by  alum  than 
the  others. 

Soda  Ultramarine  low  in  Sulphur,  which  is  a  pure  blue 
variety,  darker  and  with  more  covering  power  than  the 
preceding. 

Soda  Ultramarine  rich  in  Sulphur  and  Silica,  which 
is  the  darkest  variety  and  has  a  reddish  tinge.     It  has  high 
covering  power,  and  being  the  least  affected  by  alum  it  is 
much  used  in  paper  blueing, 
c.  6 


82  SILICA   AND   THE  SILICATES 

Ultramarine  is  formed  by  calcining  a  mixture  of 
aluminium  silicate  and  sodium  sulphide.  In  practice  China 
clay  (sometimes  with  addition  of  silica)  is  used  for  the 
former,  and  the  sodium  sulphide  is  produced  during  the 
process  of  manufacture  from  either  sodium  carbonate  or 
sodium  sulphate,  with  sulphur  and  carbon.  With  the  best 
materials,  the  following  mixtures  are  said  to  give  the  best 
results  for  the  three  varieties  of  ultramarine  blue 
mentioned  : — 

Silica 
China  clay.  Soda.      Glauber  salt.     Carbon.         (kieselguhr).         Sulphur. 

Pale  . .  100  9  120  25  o  16 
Medium  100  100  o  12  o  60 
Dark  . .  100  103  o  4  16  117 

If  the  clay  used  varies  in  composition  the  proportions  of 
clay  and  silica  should  be  adjusted. 

All  ultramarine  blues  rich  in  silica  are  prepared  by  the 
direct  method  ;  most  of  those  poor  in  silica  are  prepared  by 
the  indirect  method,  in  which  ultramarine  green  forms  an 
intermediate  product.  These  are  both  dry  processes,  and 
though  wet  methods  have  been  proposed,  they  have  ap- 
parently not  been  adopted  on  a  large  scale. 

Raw  Materials.  — China  clay  and  certain  foreign  pottery 
clays  are  used,  and  always  need  levigation  so  that  the  clay 
may  be  in  a  very  finely  divided  state  ;  it  should  also  contain 
very  little  iron  and  manganese  oxides.  Soda  is  used  in  the 
form  of  the  best  soda  ash.  Glauber  salt  should  be  free  from 
acid  and  iron,  well  calcined,  and  ground  before  use.  Sulphur 
is  generally  used  in  the  form  of  rod  or  stick  (but  powdered), 
as  poorer  forms  of  sulphur  give  dirty  tints.  Carbon  should 
not  have  above  4  per  cent,  of  moisture,  the  charcoal  from 
pine  trunks  being  preferred,  of  course  finely  powdered  ; 
pitch,  tar,  resin,  etc.,  sometimes  replace  charcoal ;  when 
coal  is  used,  it  should  be  free  from  sand,  stones,  and  pyrites. 
Silica  is  generally  used  as  kieselguhr,  but  sometimes  as 
quartz,  the  latter  being  calcined,  quenched  in  water,  dried, 
and  ground.  Silicious  carbon  (as  charred  rice  husks,  etc.) 
may  be  substituted  for  corresponding  amounts  of  carbon 
and  silica. 


SILICATES  83 

Direct  Process. — The  well-mixed  raw  materials  are 
roasted  with  access  of  air.  The  series  of  operations  include 
(in  order)  mixing,  roasting,  lixiviating,  wet-grinding,  levi- 
gating, pressing,  drying,  and  sifting.  The  roasting  should 
always  take  place  as  soon  as  the  mixtures  are  prepared,  and 
the  great  aim  is  to  secure  uniform  heating  of  the  intimately 
mixed  materials.  Different  types  of  furnace  are  used 
according  to  the  magnitude  of  the  charge.  The  heat  is 
raised  slowly  to  a  bright  red,  and  held  for  12  to  18  hours, 
or  until  the  change  is  completed  as  indicated  by  a  sample 
drawn.  The  yellowish  grey  mixture  becomes  in  turn 
brown,  green,  and  blue,  the  brown  and  green  being  both 
unstable.  The  roasting  operation  is  followed  by  slow 
cooling  (for  a  week  or  ten  days).  About  one-third  of  the 
original  weight  is  lost.  Shaft  furnaces  are  used  for  a  large 
output,  and  a  number  of  special  furnaces  have  been  patented. 

A  patented  rapid  direct  process  consists  in  heating 
quickly  to  6oo°-70o°  a  mixture  of  35  parts  China  clay, 
10  carbon,  40  sodium  alum,  8  resin,  and  7  sulphur,  keeping 
at  that  temperature  about  three  hours,  then  raising  to  900°, 
stirring  well  for  an  hour  to  facilitate  oxidation ;  powdered 
sodium  chlorate  is  next  added,  the  mixture  is  stirred  for 
15  minutes,  and  finally  there  is  a  further  30  minutes' 
heating ;  very  fine  ultramarine  is  said  to  be  produced  in 
this  way. 

In  Curtius'  patented  continuous  process,  cast-iron 
retorts  (similar  to  gas  retorts)  lined  internally  with  fire- 
proof cement,  are  used. 

Indirect  Process. — Here  the  successive  stages  are  mix- 
ing, calcining  for  ultramarine  green,  crushing,  roasting  to 
ultramarine  blue,  and  then  lixiviating,  etc.,  as  in  the 
direct  process. 

The  ultramarine  green  is  prepared  in  a  crucible  furnace 
or  a  shaft  furnace,  for  smaller  and  larger  quantities  re- 
spectively. 

The  crushed  ultramarine  green  is  roasted  with  sulphur 
to  convert  it  into  blue.  The  operation  is  carried  out  largely 
in  cylindrical  or  retort  furnaces,  and  muffle  furnaces.  The 


84  SILICA   AND   THE  SILICATES 

sulphur  may  be  added  before  the  ultramarine  green  is  heated, 
or  added  gradually  during  the  heating.  With  green  made 
from  soda,  7  per  cent,  of  the  weight  of  the  crude  green  is 
required,  and  with  Glauber  salt  green  9  to  10  per  cent. 

In  making  the  cheaper  brands  of  ultramarine  blue,  the 
pure  material  is  mixed  with  10  to  50  per  cent,  of  gypsum, 
which  greatly  reduces  the  brightness  of  certain  kinds  of 
ultramarine ;  this  effect  is  minimized  by  incorporating  a 
little  glycerine  or  vaseline  into  the  mixture. 

When  first  manufactured,  artificial  ultramarine  was 
about  1 6s.  per  lb.,  but  this  was  soon  reduced  through 
competition. 

In  1913  about  100  factories  were  at  work,  mostly  in 
Germany  and  France,  with  a  few  in  England,  Austria,  and 
the  United  States.  The  average  price  was  less  than  305.  per 
cwt.  for  an  annual  production  which  had  reached  to  between 
10,000  and  15,000  tons. 


MICA. 

Mica  comprises  a  group  of  rock-forming  minerals  which 
occur  frequently  in  nature.  Some  of  them  have  commercial 
applications  of  considerable  importance.  The  chief  micas 
are  muscovite,  biotite,  phlogopite,  and  lepidolite. 

The  general  characters  of  the  micas  include  a  very  easy 
cleavage  in  a  single  direction  (so  that  they  split  up  into 
very  thin  sheets),  and  the  great  flexibility,  elasticity,  and 
toughness  of  the  cleavage  flakes.  They  all  crystallize  in 
the  monoclinic  system.  The  colour  varies  from  perfectly 
colourless  and  transparent  (as  in  muscovite)  through  shades 
of  yellow,  green,  red,  and  brown  to  black  and  opaque. 
The  colourless  and  transparent  varieties  have  a  pearly 
lustre  on  the  flat  surfaces,  whereas  something  approaching 
a  metallic  lustre  is  found  on  the  others. 

The  hardness  of  mica  is  2  to  3  (the  smooth  surfaces  can 
be  just  scratched  with  the  finger  nail),  and  the  specific 
gravity  ranges  from  27  to  3-1  in  the  different  species. 


SILICATES  85 

They   are   poor   conductors   of  heat   and   electricity,    and 
therefore  are  useful  for  insulating  purposes. 

The  chemical  composition  of  micas  exhibits  great 
complexity.  They  are  usually  orthosilicates  of  aluminium 
with  alkali  metals,  basic  hydrogen,  and  in  some  species 
magnesium  and  iron  (ferrous  and  ferric),  and  rarely  other 
metals.  Fluorine  is  often  an  essential  constituent. 

Micas  may  be  subdivided  into  two  groups :  alkali 
micas,  including  muscovite,  paragonite,  and  lepidolite  ; 
and  ferro-magnesian  micas,  comprising  biotite,  phlogopite, 
and  other  less  important  varieties.  In  a  rough  way,  these 
two  groups  answer  to  the  light  and  dark-coloured  micas 
respectively.  Biotite  is  usually  a  deep  black,  but  in  thin 
films  is  dark  brown,  or  yellow,  or  a  hazy  green. 

The  water  present  in  micas — 4  to  6  per  cent,  in  muscovite, 
and  scarcely  as  much  in  the  others — can  only  be  driven  off 
at  a  high  temperature,  and  is  therefore  water  of  constitution, 
not  merely  water  of  hydration. 

Artificial  crystals  of  micas  have  been  observed  in  furnace 
slags,  and  in  silicate  fusions. 

The  composition  of  muscovite  is  perhaps  most  simply 
represented  by  the  formula  H^KA^SiO^.  It  may  also 
be  represented  as  2SiO2.Al2O3.JK2O.fH2O,  which  shows  it 
as  differing  from  anorthite  by  the  presence  of  potash  and 
water  instead  of  an  equivalent  amount  of  lime.  A  third 
way  of  representing  the  composition  of  muscovite  is  by  the 
formula  6SiO2.3Al2O3.K2O.2H2O,  which  avoids  fractions 
of  molecules.  In  the  rare  mica  paragonite,  the  potassium 
of  muscovite  is  simply  replaced  by  sodium.  L,epidolite, 
which  seems  to  be  a  metasilicate  of  alumina  and  lithia, 
has  a  more  complex  formula,  including  some  fluorine. 

Biotite  is  essentially  an  orthosilicate  of  alumina,  magnesia, 
and  potash,  with  some  of  the  last-named  replaced  by  water, 
and  some  of  the  magnesium  and  aluminium  replaced  by 
ferrous  and  ferric  iron  respectively.  The  composition  of 
phlogopite  is  somewhat  similar,  but  with  little  iron  and 
relatively  more  magnesia,  and  some  fluorine. 

Muscovite  is  commonly  known  as  white  mica  or  potash 


86  SILICA   AND  THE  SILICATES 

mica.  The  large  sheets  of  commercially  valuable  muscovite 
are  only  met  with  in  very  coarsely  crystalline  veins  of 
pegmatite  running  through  granite,  gneiss,  or  mica  schist. 
Such  veins  are  worked  for  mica  in  India,  the  United  States, 
and  Brazil.  In  Canada  and  Ceylon  the  micas  of  commercial 
value  are  chiefly  phlogopite.  In  India,  "  books  "  of  mica 
have  been  obtained  as  much  as  10  to  15  ft.  across,  and 
sheets  30  X24  ins.,  without  cracks  or  flaws. 

Uses  of  Mica. — Its  transparency,  and  its  resistance  to 
fire  and  to  sudden  temperature  changes,  has  led  to  the 
extensive  use  of  mica  for  the  windows  of  stoves  and  lanterns, 
peep-holes  of  furnaces,  and  lamp  chimneys  and  gas  burners. 
It  was  formerly  used  for  the  windows  of  houses,  and  in  the 
port  holes  of  Russian  war  vessels.  Spangles  of  mica  are 
largely  employed  for  decorative  purposes.  Powdered  mica 
is  very  much  used  in  the  manufacture  of  wallpaper  and 
other  papers,  for  "  frosting  "  toys  and  stage  scenery,  etc., 
for  the  manufacture  of  paints,  as  a  lubricant,  and  for 
absorbing  nitroglycerine  and  disinfectants.  Sheets  of  mica 
are  used  for  painting  on,  for  making  lantern  slides,  for 
supporting  photographic  films,  for  covering  pictures  and 
valuable  documents.  Other  uses  are  for  supports  for 
preserved  natural  history  specimens  (in  spirit),  for  mirrors 
of  delicate  scientific  instruments,  for  vanes  of  anemometers, 
and  for  a  variety  of  optical  and  other  purposes.  Its  in- 
sulating properties  as  regards  heat  make  it  valuable  for  the 
packing  and  jackets  of  boilers  and  steam  pipes.  The 
resistance  offered  by  mica  to  acids  also  finds  useful 
application. 

The  most  extensive  application  of  mica  is  for  electrical 
insulation.  The  smooth  and  flexible  mica  sheets  are  in 
great  demand  for  electrical  machinery,  including  armatures 
of  dynamos.  Small  sheets  of  mica  cemented  by  means  of 
an  insulating  cement  (as  shellac)  on  paper  or  cloth  finds 
various  uses  under  the  name  of  "  micanite  "  or  "  micanite 
cloth/' 

Phlogopite  seldom  occurs  in  colourless,  transparent 
sheets,  so  its  use  is  nearly  confined  to  electrical  insulation. 


SILICATES  87 

The  mica  mined  in  Canada  and  Ceylon  is  mostly  phlogopite, 
and  is  used  chiefly  for  insulation.  The  colour  of  phlogopite 
is  dark  grey,  smoky,  or  yellowish  to  brown-red — more 
usually  yellowish-brown — the  latter  being  known  as  amber 
mica  in  commerce,  or  "  silver  amber  mica  "  in  allusion  to 
the  frequent  silvery  lustre  on  the  cleavage  surfaces. 

Muscovite  is  of  more  frequent  occurrence,  and  of  more 
general  application.  Other  kinds  of  mica  (than  muscovite 
and  phlogopite)  are  scarcely  used  commercially. 

Mica  has  some  value  as  a  fertilizer  because  of  the  potash 
it  contains. 

A  bright  green  variety  of  muscovite  called  fuchsite,  or 
chrome-mica,  has  been  used  as  an  ornamental  stone,  and  also 
lepidolite  or  lithia  mica. 

In  dressing  mica  the  "  books  "  are  split  into  sheets  of 
suitable  thickness,  and  the  sheets  trimmed  with  a  sharp 
knife,  shears,  or  guillotine.  The  dressed  sheets  are  sorted 
according  to  size,  transparency,  colour,  and  freedom  from 
spots  and  stains.  Scrap  mica  is  ground,  or  used  for  making 
micanite. 

Chlorite  is  a  hydrated  silicate  of  magnesium  and 
aluminium,  with  the  two  metals  partly  replaced  by  ferrous 
and  ferric  iron  respectively.  The  ferrous  oxide  often 
amounts  to  8  or  9  per  cent.,  or  even  more.  The  colour  is 
green,  yellowish,  or  white,  the  hardness  i  to  ij,  and  specific 
gravity  about  2 '8.  It  is  usually  foliated  and  scaly ;  massive 
chlorite  is  amorphous  and  darker  in  colour.  Earthy  chlorite 
consists  of  small  scaly  particles  with  a  somewhat  greasy 
feel.  The  laminae  are  flexible,  but  not  elastic  like  those  of 
mica.  At  Portsoy,  in  Banff  shire,  chlorite  is  mixed  with 
serpentine,  and  is  often  cut  and  polished,  notwithstanding 
its  softness. 

Tourmaline  is  a  well-crystallized  mineral  of  rather 
complex  composition,  roughly  represented  by  2SiO2.Al2O3.- 
MgO.O'33B2O3.  It  is  a  boro-silicate  of  aluminium,  with 
variable  amounts  of  ferrous  oxide,  magnesia,  and  alkalies. 
Tourmalines  are  closely  associated  with  micas.  The  colour 
ranges  from  colourless  to  jet  black,  and  may  be  any  shade  of 


88  SILICA   AND   THE  SILICATES 

red,  yellow,  green,  or  blue.  Some  crystals  show  different 
colours  in  different  parts.  The  opposite  ends  of  crystals  are 
usually  different.  Crystals  are  often  triangular,  from  suppres- 
sion of  alternate  faces.  The  crystal  faces  are  generally  long 
and  striated.  The  hardness  is  7  to  7^,  and  specific  gravity 
2 '94  to  3*3.  Coloured  transparent  tourmalines  are  used 
for  gems,  especially  rubellite  or  red  tourmaline.  Another 
use  is  in  the  tourmaline  pincette,  consisting  of  two  mounted 
transparent  plates  of  tourmaline  cut  parallel  to  the  principal 
axis ;  it  is  used  for  the  examination  of  crystal  sections  in 
convergent  polarized  light.  Black  tourmaline  is  called 
schorl,  the  red  is  rubellite,  and  the  indigo  blue  is  indicolite. 
Tourmaline  is  pyroelectric.  It  is  usually  found  in  granite, 
gneiss,  mica  slate,  certain  limestones  and  sandstones. 

Axinite  is  a  complex  aluminium  and  calcium  boro- 
silicate,  the  aluminium  being  partly  replaced  by  ferric 
iron,  and  the  calcium  by  ferrous  iron  and  manganese,  etc. 
The  name  alludes  to  the  wedge-shaped  crystals.  The  colour 
is  mostly  clove-brown  or  violet,  but  different  colours  may 
be  seen  in  different  directions  (as  cinnamon-brown,  violet 
blue,  olive-green).  The  hardness  is  6J  to  7,  and  the  specific 
gravity  3*28.  The  hardness,  colour,  and  transparency 
enable  axinite  to  be  used  as  a  gem-stone,  and  it  is  sometimes 
so  used,  but  it  is  easily  fractured.  It  occurs  in  Cornwall 
and  Devonshire,  but  the  best  crystals  are  found  in  Dauphine 
(France). 

Epidote  is  a  basic  calcium,  aluminium,  and  iron  ortho- 
silicate,  6vSiO2.3(Al,Fe)2O3.4CaO.H2O,  containing  5  to  17 
per  cent,  of  ferric  oxide.  It  has  the  same  ratio  of  silica  to 
alumina  (with  ferric  oxide)  as  in  anorthite,  etc.  Some  of  its 
characters  vary  with  the  proportion  of  iron.  The  hardness 
is  6J,  specific  gravity  3*3  to  3-5,  colour  usually  a  yellowish- 
green,  but  sometimes  green,  grey,  brown,  or  nearly 
black. 

Epidote  is  an  abundant  rock-forming  mineral,  but  is  of 
secondary  origin,  having  been  formed  during  metamorphic 
changes  in  the  rock.  It  occurs  in  many  schists  and  crystal- 
line limestones,  and  also  results  from  the  weathering  of 


SILICATES  89 

felspars,  micas,  garnets,  pyroxenes,  amphiboles,  and  other 
mineral  constituents  of  igneous  rocks. 

Transparent  dark-green  crystals  of  epidote  have  some- 
times been  cut  as  gem-stones. 

Zoisite  closely  resembles  epidote,  from  which  it  differs 
chiefly  in  containing  little  or  no  iron.  The  same  formula 
serves,  if  the  Fe  be  omitted.  Hardness  6J,  and  specific 
gravity  about  3*3.  The  colour  is  mostly  white  or  grey. 
A  Norwegian  rose-red  variety,  called  thulite,  has  been  used  to 
some  extent  as  an  ornamental  stone. 


AlyUMINOTRISIUCATES . 

To  this  class  belong  natrolite  and  several  other  zeolites, 
and  garnets.  Natrolite  is  one  of  the  commonest  of  the 
group  of  zeolites,  and  has  a  composition  represented 
by  Na2Al2Si3O10.2H2O,  or  3SiO2.Al2O3.Na2O.2H2O.  It  is 
usually  found  as  radiating  fibrous  masses  in  the  steam 
cavities  of  volcanic  rocks.  Hardness  5  to  5j,  and  specific 
gravity  about  2*25  ;  usually  colourless  or  white,  but  some- 
times greyish,  yellowish,  or  reddish-brown.  A  German 
compact  mass  of  radiating  fibres  with  concentric  yellow 
bands  has  sometimes  been  cut  and  polished  for  ornaments. 


GARNETS. 

The  garnets  are  alumino-trisilicates  of  lime,  magnesia, 
ferrous  oxide,  etc.,  which  crystallize  in  the  cubic  system, 
and  form  isomorphous  mixtures  in  which  lime,  magnesia, 
ferrous  oxide  or  manganese  oxide  are  mutually  replaceable, 
and  in  which  the  alumina  may  be  partly  or  wholly  replaced 
by  ferric  or  chromic  oxide. 

In  ancient  times  garnets  were  used  as  beads,  and  for 
making  engraved  gems. 

The  general  formulae  of  garnets  is  R3"R2'"(SiO4)3,  all 
being  orthosilicates  in  which  R"=Ca,  Mg,  Fe",  or  Mn,  or 
more  than  one  of  them,  and  R'"=A1,  Fe"',  or  Cr. 


go  SILICA   AND   THE  SILICATES 

Garnets  may  be  grouped  under  six  types  as  follows  : — 

1.  Lime-alumina  garnet,  Ca3Al2(SiO4)3,  or  3SiO2,Al2O3.- 
3CaO,  including  grossularite,  essomte,  or  cinnamon  stone,  etc. 

2.  lyime-iron  garnet,  Ca3Fe2(SiO4)3,  or  3SiO2.Fe2O3.CaO, 
including  andradite,  etc. 

3.  Lime-chromium  garnet,  Ca3Cr2(SiO4)3,  or  3SiO2.Cr2O3.- 
3CaO,  including  uwarowite. 

4.  Magnesia-alumina  garnet,   Mg3Al2(SiO4)3,   or  3SiO2.- 
Al2O3.3MgO,  or  pyrope. 

5.  Iron-alumina  garnet,  Fe3Al2(SiO4)3,  or  3SiO2.Al2O3.- 
3FeO,   including  almandine   (precious  or  noble  garnet)   or 
carbuncle. 

6.  Manganese-alumina  garnet,  Mn3Al2(SiO4)3,  or  3SiO2.- 
Al2O3>3MnO,  including  spessartite. 

The  types  are  usually  modified  by  isomorphous  replace- 
ments. 

The  hardness  of  garnets  ranges  from  6J  to  7},  and  the 
specific  gravity  also  varies  from  3*4  (in  lime-alumina  garnets) 
to  4-3  (in  iron-alumina  garnets).  The  colour  is  often  red, 
but  may  be  brown,  yellow,  green,  or  even  black  or  colourless. 

Garnets  are  very  widely  distributed,  occurring  in  crystal- 
line schists,  gneiss,  granite,  serpentine,  and  certain  altered 
limestones,  etc.  Garnets  used  for  industrial  purposes  are 
mostly  found  loose,  weathered  from  the  rock  in  which  they 
occurred,  but  sometimes  the  rock  is  quarried  for  garnets. 

Jewellers  class  garnets  as  Syriam  or  oriental  (so-called 
from  Syriam,  the  capital  of  Pegu),  Bohemian,  or  Cingalese, 
according  to  relative  fineness  rather  than  place  of  origin. 
The  most  valued  garnets  are  the  Syriam,  of  violet-purple 
colour  (unmixed  with  black),  distinguished  from  oriental 
amethyst  in  being  orange-tinted  by  candle-light.  Bohemian 
garnet  is  generally  of  a  dull  poppy-red  colour,  with  distinct 
hyacinth-orange  when  placed  between  the  eye  and  the  light. 
When  of  a  full  crimson  colour,  the  stone  is  called  pyrope  or 
fire-garnet,  which  is  valuable  when  large  and  perfect.  Cut 
garnets  hollowed  out  on  the  under  side,  and  set  with  silver 
foil,  have  often  been  sold  as  rubies.  Melanite,  a  black 
variety  of  andradite,  was  formerly  used  for  mourning 


SILICATES  91 

jewellery.  Spessartite  is  a  fine  aurora-red  garnet,  cut  for 
jewellery  when  clear  enough,  and  somewhat  resembles 
cinnamon-stone.  A  variety  of  essonite  or  cinnamon-stone 
known  as  romanzowite,  from  Finland,  is  often  polished  and 
sold  as  hyacinth,  the  true  hyacinth  being  a  reddish  variety 
of  zircon.  A  rose-red  American  garnet  called  rhodolite 
forms  a  fine  gem-stone. 

Besides  being  cut  as  gems,  garnets  are  used  for  the 
bearings  of  pivots  in  watches.  The  coarser  garnets  have 
been  used  in  powdered  condition  as  abrasives,  for  which 
purpose  it  is  superior  to  sand,  though  inferior  to  emery  in 
hardness.  The  powder  is  also  used  for  cutting  gems. 
Garnet  paper  is  largely  used  instead  of  sand-paper  for 
smoothing  woodwork,  and  for  scouring  leather  in  the  boot 
trade. 

Al,UMINO-HEXASIIJCATES. 

Felspars  form  a  very  important  group  of  mineral 
silicates.  Orthodase  or  potash  felspar,  6SiO2.Al2O3.K2O, 
and  albite  or  soda  felspar,  6SiO2.Al2O3.Na2O,  are  the  most 
abundant  silicates  occurring  in  such  old  crystalline  rocks  as 
granite  and  gneiss.  They  give  mixed  crystals  with  the 
alumino-disilicate,  anorthite,  2SiO2.Al2O3.CaO,  which  on 
this  account  is  classed  with  the  felspars.  Albite  is  usually 
white,  as  the  name  implies.  Orthodase  has  a  hardness  of 
6,  and  specific  gravity  about  2-56  ;  the  crystals  show  two 
well-marked  cleavages  at  right  angles,  whereas  the  two 
corresponding  cleavages  in  other  felspars  are  not  rectangular. 
Occasionally  orthoclase  is  colourless,  but  it  is  generally- 
white,  red,  yellow,  grey,  or  green.  Varieties  of  orthoclase 
include  :  (i)  Common  felspar,  which  is  generally  white  or 
red ;  also  the  yellowish-grey  or  flesh-red  murchisonite ; 

(2)  adularia  and  ice-spar,  transparent  or  translucent,  and 
nearly  colourless,  including  moonstone  with  bluish  opalescence ; 

(3)  sanidine  or  glassy  felspar. 

Microcline  resembles  orthoclase  in  composition,  but  is 
like  the  other  felspars  in  crystallographic  characters.  The 


92  SILICA   AND  THE  SILICATES 

bright  verdigris-green  A mazon  stone  of  Sutherland,  etc.,  is  a 
variety  of  microcline. 

Albite  or  soda  felspar  forms  mixed  crystals  with  anorthite 
or  lime-felspar  in  various  proportions  to  form  a  number  of 
intermediate  felspars,  the  series  in  order  including  albite, 
oligoclase,  andesine,  labradorite,  bytownite,  and  anorthite. 

Several  varieties  of  felspar  have  been  cut  as  ornamental 
stones,  including  the  moonstone  (adularia),  the  green 
Amazon  stone,  and  labradorite ;  sunstone  or  aventurine 
felspar  (oligoclase)  and  occasionally  colourless,  transparent, 
glassy  felspar  have  been  used  as  gem-stones. 

Orthoclase  felspar  is  largely  used  for  the  manufacture 
of  porcelain  and  stoneware,  and  for  ceramic  glazes.  For 
these  purposes  it  is  crushed  and  ground  before  being  mixed 
with  the  other  ingredients.  Other  felspars  have  been 
similarly  used  to  a  small  extent,  especially  albite.  Ortho- 
clase has  also  been  used  as  a  source  of  potassium  compounds. 


FELSPAR  AND  OTHER  SlUCATE  MINERALS  AS  SOURCES 
OF  POTASH. 

The  comparative  abundance  of  certain  natural  silicates 
was  bound  sooner  or  later  to  excite  speculation  as  to  the 
possibility  of  extracting  potash  from  them.  As  far  back 
as  1857,  F.  O.  Ward  took  out  a  patent  (No.  3185,  Dec.  30) 
for  the  preparation  of  potassium  and  sodium  compounds 
from  such  materials  as  orthoclase,  albite,  pumice,  lava, 
etc.,  by  mixing  them  with  calcium  fluoride  and  lime  (or 
calcium  carbonate)  and  heating  to  bright  redness ;  the 
alkali  was  extracted  from  the  frit  by  lixiviation  with  boiling 
water.  Since  then  many  other  methods  have  been  devised 
for  the  purpose.  It  is  known  that  simple  wet  grinding 
extracts  some  of  the  potash,  but  less  than  0-5  per  cent.,  and 
even  enormous  pressure  does  not  enable  water  to  decompose 
felspar  readily.  By  digesting  felspar  and  lime  (in  the 
proportion  of  i  :  17)  with  water  under  a  steam  pressure  of 
10  to  15  atmospheres,  W.  H.  Ross  claims  that  about  90  per 
cent,  of  the  potash  is  dissolved,  the  insoluble  residue  being 


SILICATES  93 

suitable  for  making  Portland  cement.  Generally  speaking, 
it  may  be  said  that  a  process  can  only  be  successful  com- 
mercially when  the  residue  can  be  usefully  employed. 

Attempts  have  been  made  to  solve  the  problem  by  the 
use  of  acids  as  well  as  alkaline  substances ;  among  others 
hydrofluosilicic  acid  followed  by  sulphuric  acid,  or  dilute 
hydrofluoric  acid.  Above  100°  hydrofluoric  acid  acts  as  a 
catalyst  in  presence  of  sulphuric  acid,  sulphates  of  aluminium 
and  potassium  being  formed.  Spiller  proposed  to  produce 
the  same  sulphates  by  heating  felspar  with  calcium  fluoride 
and  sulphuric  acid,  and  a  somewhat  similar  method  has 
been  patented  by  Hone,  who  collects  the  escaping  silicon 
fluoride  to  form  a  fluosilicate,  which  in  turn  can  be  used  in 
place  of  simple  fluoride  with  more  of  the  mineral. 

Hart  obtains  sulphates  (potash  alum)  by  fusing  orthoclase 
with  the  proper  quantity  of  barium  sulphate  and  coal,  and 
acting  on  the  powdered  products  with  sulphuric  acid. 

Thompson  claims  that  80  to  90  per  cent,  of  the  potassium 
in  felspar  can  be  obtained  as  sulphate  from  such  mixtures 
as  5  parts  each  of  felspar  and  acid  sodium  sulphate,  and 
1*8  parts  of  sodium  chloride,  by  heating  to  redness  for  2  or 
3  hours,  grinding  the  cooled  product,  lixiviating,  and 
crystallizing  out. 

Ashcroft  treats  crushed  felspar,  etc.,  with  fused  sodium 
chloride  (or  calcium  chloride)  excluding  air  and  moisture, 
and  thus  obtains  potassium  chloride.  In  another  patented 
process  he  heats  felspars,  etc.,  with  excess  of  potassium 
chloride  (or  sulphate,  or  nitrate)  to  replace  soda  by  potash, 
and  washes  out  the  soluble  salts ;  the  product  is  treated  with 
lime  (or  magnesia,  or  both)  and  water  or  steam  to  get 
caustic  potash  and  a  residue  suitable  for  making  cement. 
In  a  more  recent  patent,  Ashcroft  treats  felspar,  etc.,  in  a 
converter  at  about  800°  to  1100°  C.  with  chlorine  gas  while 
in  a  bath  of  fused  sodium  chloride  (or  potassium  chloride, 
or  both).  Potassium  chloride  is  separated  by  lixiviation. 

Scholes  fuses  a  mixture  of  felspar,  etc.,  and  alkali 
compounds.  The  product  is  ground,  boiled  with  water, 
and  treated  with  carbonic  acid,  when  alkaline  carbonate 


94  SILICA   AND  THE  SILICATES 

is  left  in  solution,  while  hydrated  silica  and  alumina,  etc., 
are  precipitated. 

Potassium  carbonate  may  thus  be  obtained  by  roasting 
mixtures  of  felspar,  etc.,  with  calcium  compounds  (carbonate, 
hydroxide,  etc.),  and  blowing  steam  through  the  product, 
and  in  this  case  the  residues  might  be  made  suitable  for 
making  Portland  cement. 

In  other  processes,  felspar  is  fritted  with  salt,  with  or 
without  lime,  to  give  potassium  chloride,  and  in  some  cases 
(where  calcium  compounds  are  used)  the  residue  is  suitable 
for  making  Portland  cement.  In  some  processes,  the  potash 
or  potassium  chloride  is  volatilized  and  collected  by  condensa- 
tion ;  this  is  especially  the  case  with  cement  kiln  dusts  and 
blast-furnace  dusts.  During  the  war,  K.  M.  Chance  and 
others  averted  a  serious  deficiency  by  combining  extraction 
of  potassium  chloride  with  purification  of  blast-furnace 
gases ;  sodium  chloride  was  added  to  the  furnace  charges 
to  convert  the  potassium  present  into  chloride. 

In  some  instances  the  product  is  adapted  for  use  for 
fertilizing  purposes. 

In  Frazer,  Holland,  and  Miller's  process,  felspar  is  mixed 
with  potassium  hydroxide  or  sodium  hydroxide,  and  the 
dried  mixture  heated  for  an  hour  at  300°  C.,  then  treated 
with  water  and  filtered.  The  alkali  used  may  be  recovered 
from  the  filtrate.  The  insoluble  part,  consisting  of  an 
artificial  leucite,  KAl(SiO3)2,  is  treated  with  hydrochloric 
acid  to  form  potassium  chloride,  and  the  insoluble  aluminium 
silicate  is  easily  decomposed  by  sulphuric  acid,  producing 
aluminium  sulphate. 

ZEOUTES. 

The  Zeolites  form  a  group  of  minerals  which  are  hydrated 
compounds  consisting  of  silica,  alumina,  and  alkaline  or 
alkaline  earthy  bases  or  both.  They  readily  give  up  their 
water — though  in  some  cases  part  of  it  is  only  lost  at  a  red 
heat — and  when  they  are  heated  by  the  blow-pipe  flame  its 
expulsion  is  accompanied  by  swelling  (hence  the  name  zeolite, 


SILICATES  95 

from  Greek  zein,  "to  boil/'  and  lithos,  "a  stone ").  Their 
water  can  be  replaced  by  ammonia,  alcohol,  etc.  They  are 
all  readily  soluble  in  hydrochloric  acid,  and  generally  with 
separation  of  gelatinous  silica.  In  some  cases  the  composi- 
tion of  the  same  mineral  varies  to  a  certain  extent,  which  may 
be  regarded  as  indicating  the  presence  of  isomorphous 
mixtures  of  different  molecules.  The  zeolites  are  mostly 
glassy  looking  or  white,  but  sometimes  red,  grey,  or  yellow. 
They  generally  occur  as  crystals  (often  with  agate,  calcite, 
etc.)  lining  cavities  or  fissures  in  basalts  and  other  crystalline 
igneous  rocks,  as  though  they  were  deposited  from  percolating 
water,  and  they  probably  result  from  decomposition  of 
nepheline,  or  sodalite,  or  felspars.  They  often  form  fine 
crystals,  but  rarely  constitute  rock-forming  minerals. 

Analcite,  NaAlSi2O6  -f  H2O,  or  4SiO2.  Al2O3.Na2O  +2H2O, 
has  been  found  in  the  form  of  irregular  grains  as  a  primary 
constituent  of  certain  basalts  and  other  igneous  rocks. 

The  hardness  of  zeolites  ranges  between  3!  and  5j, 
and  the  specific  gravity  between  2*0  and  2*4,  but  rather  low. 

When  a  partially  dehydrated  and  opaque  crystal  is 
exposed  to  moist  air,  water  is  reabsorbed,  and  the  crystal 
regains  its  transparency  and  optical  characters. 

Prehnite,  H2Ca2Al2(SiO4)3,  or  3SiO2.Al2O3.2CaO.H2O, 
unlike  analcite  and  ordinary  zeolites,  loses  its  water 
(amounting  to  between  4  and  5  per  cent.)  only  at  a  red  heat. 
Its  hardness  is  a  little  over  6,  specific  gravity  about  2*9,  and 
colour  usually  pale  green.  Prehnite  is  sometimes  cut  and 
polished  for  small  ornaments,  and  then  appears  rather  like 
chrysoprase. 

Among  the  more  important  zeolites  are  natrolite,  a 
sodium,  zeolite  (already  noticed  among  alumino-trisilicates, 
immediately  before  the  garnets),  and  thomsonite,  chabazite, 
heulandite,  and  stilbite,  all  calcium  zeolites. 

The  natural  zeolites  themselves  have  no  direct  industrial 
importance,  but  an  indefinite  zeolitic  component  of  soils, 
known  as  geolyte,  seems  to  give  valuable  service  to  agri- 
culture by  retaining,  in  a  form  easily  soluble,  the  potash 
and  soda  released  when  felspar  is  decomposed  on  weathering. 


96  SILICA   AND  THE  SILICATES 

Artificial  zeolites  have  been  prepared,  including  a  sodium 
alurninium  silicate  called  permutite,  which  is  now  largely 
used  for  softening  water,  and  in  sugar  refining. 


WATER  SOFTENING  BY  THE  PERMUTITE  PROCESS. 

Though  only  introduced  about  ten  years  ago,  by  Dr.  Cans 
of  Berlin,  this  process  is  coming  more  and  more  into  use  for 
industrial  purposes,  because  of  its  simplicity  and  effective- 
ness. It  has  the  great  advantages  of  not  requiring  the 
addition  of  chemicals,  and  of  avoiding  the  accumulation  of 
sludge  or  slime.  The  hard  water  (sometimes  after  partial 
softening)  is  simply  passed  through  a  bed  of  granulated 
permutite.  It  resembles  a  filtration  process,  but  differs 
materially  from  ordinary  filtration  in  the  fact  that  no  solid 
matter  is  deposited  ;  indeed  it  is  necessary  for  the  water  to 
be  quite  clear  when  subjected  to  this  treatment. 

Permutite  (or  sodium  permutite)  has  a  composition 
represented  by  the  formula  zSiC^.A^C^.Na^.GH^O,  or 
Na2Al2Si2O8.6H2O,  and  is  practically  insoluble  in  water. 
It  is  now  manufactured  (at  Brentford)  from  sodium  silicate 
and  sodium  aluminate,  and  is  supplied  in  the  form  of  hard 
granules  resembling  coarse  quartz.  Permutite  has  also 
been  made  by  melting  together  a  mixture  of  sodium 
carbonate,  China  clay,  or  alumina,  and  silica  (as  quartz, 
etc.).  During  the  passage  through  this  material  of  water 
containing  bicarbonate  or  sulphate  of  calcium  or  magnesium, 
or  salts  of  iron  or  manganese,  the  sodium  of  the  permutite  is 
exchanged  (whence  the  name)  for  the  metal  of  the  dissolved 
salt:— 

Na2Al2Si208.6H20+CaH2(C03)2=CaAl2Si208.6H20+2NaHC03 
Na2Al2Si2O8.6H2O+MgSO4=CaAl2Si2O8.6H2O+Na2SO4 

All  traces  of  calcium  and  magnesium  are  removed. 

When  the  permutite  ceases  to  be  effective  because  of 
substitution  of  alkaline  earthy  metal  for  sodium,  or  rather 
before  that  stage  is  reached,  its  activity  can  be  renewed — 
without  removing  the  material — by  washing  it  for  about 
10  hours  with  a  10  per  cent,  solution  of  sodium  chloride 


SILICATES  97 

(which  must  be  free  from  magnesium  chloride),  the  converse 
change  taking  place.  According  to  the  hardness  of  the 
water  and  the  quantity  of  permutite  in  use,  a  certain  quantity 
of  the  hard  water  is  passed  through  before  the  permutite 
is  regenerated.  This  avoids  the  risk  of  failure  through 
going  on  too  long.  It  has  been  found  that  water  with 
much  magnesium  requires  more  permutite  than  water 
containing  calcium  salts.  The  calcium  and  magnesium 
chlorides  formed  during  the  regeneration  are  washed 
away. 

Before  the  introduction  of  the  salt  solution,  the  permutite 
is  agitated  for  a  few  minutes,  and  so  loosened,  by  water 
being  made  to  enter  the  bottom  of  the  filtering  bed,  any 
air  channels  formed  during  the  softening  process  being 
destroyed  at  the  same  time.  The  salt  solution  percolates 
slowly  downwards  through  the  permutite,  and  after  about 
10  hours  the  remaining  salt  solution  is  washed  out ;  the 
softening  process  can  then  begin  again. 

When  very  hard  water  requires  thorough  softening,  the 
ideal  plan  would  be  to  remove  the  bulk  of  the  hardening 
substances  by  means  of  one  of  the  ordinary  lime  and  soda 
softening  contrivances,  and  the  remaining  hardness  by  the 
permutite  method.  In  some  works  this  plan  has  failed 
after  some  years,  owing  no  doubt  to  the  alkalinity  (from 
free  lime,  sodium  carbonate,  or  caustic  soda)  of  the  water 
from  the  lime  and  soda  plant.  Free  lime  passing  through 
the  pejmutite  produces  caustic  soda,  which  will  gradually 
dissolve  it.  Sodium  carbonate  passing  through  partly 
altered  permutite  will  give  rise  to  sodium  permutite  and 
calcium  carbonate,  the  latter  being  sometimes  carried  on- 
ward to  make  the  water  look  opalescent.  Free  alkali 
causes  a  further  deposition  of  chalk  from  the  residual 
hardness.  As  is  well  known,  water  even  after  filtering 
through  wood-wool  will  on  filtering  through  sand  give  a 
further  deposit  of  chalk  (with  further  softening  of  the  water), 
the  sand  accelerating  the  softening  action.  Permutite 
acts  like  the  sand,  but  perhaps  more  energetically  because 
of  its  porosity, 
c-  7 


98  SILICA  AND  THE  SILICATES 

In  cases  where  permutite  works  successfully  after 
previous  softening,  the  water  rests  for  some  time  before 
passing  on  to  the  permutite,  thus  permitting  calcium 
carbonate  to  settle  ;  such  water  even  after  two  or  three 
days'  rest  was  still  alkaline. 

Attempts  have  been  made  to  overcome  this  difficulty 
(of  alkaline  water)  by  taking  advantage  of  the  fact  that 
calcium  permutite  can  absorb  free  lime  to  the  extent  of 
the  amount  it  originally  contains,  and  on  subsequently 
passing  hard  water  in  the  opposite  direction  the  additional 
lime  becomes  removed  as  chalk,  which  is  washed  away. 
This  method  works  well  for  a  while,  but  later  free  lime 
again  passes  through  without  being  absorbed  by  the  calcium 
permutite.  As  P.  B.  King  points  out  (Society  of  Dyers 
and  Colourists,  xxxiv.  No.  12  ;  Chemical  News,  118,  16, 
1919),  even  if  this  difficulty  should  be  overcome,  a  plant 
consisting  of  three  systems — lime  and  soda,  then  say  calcium 
permutite,  followed  by  sodium  permutite — would  probably 
cost  more  to  work  than  a  sodium  permutite  system  operating 
directly  on  the  crude  hard  water. 

In  like  manner,  for  removing  iron  and  manganese  from 
water,  Gans  employs  a  manganese-permutite,  consisting  of 
silica,  alumina,  and  a  higher  oxide  of  manganese,  the 
material  being  regenerated  as  required  by  treatment  with 
a  solution  of  potassium  or  calcium  permanganate. 
Manganese-permutite  can  also  be  applied  for  sterilizing 
water,  the  water  being  first  treated  with  potassium  per- 
manganate to  destroy  the  bacteria,  and  then  filtered  through 
manganese-permutite  so  that  the  filter  may  remove  the 
manganese. 

Calcium  Permutite  has  been  used  for  removing  potash 
from  molasses,  the  lime  being  afterwards  substituted  by 
soda  on  passing  the  solution  through  sodium-permutite. 

The  use  of  the  permutite  process  for  softening  water  has 
been  found  specially  advantageous  in  the  case  of  dye-works, 
bleacheries,  etc. 

According  to  B.  Ramann  and  A.  Spengel  (Zeits.  anorg. 
Chem.,  £5-,  115, 1916),  in  the  presence  of  small  quantities  of 


SILICATES  99 

calcium  purification  is  effected  more  rapidly  by  treating 
with  10  per  cent,  ammonium  nitrate  solution,  followed  by 
reconversion  of  the  ammonium  permutite  into  sodium 
permutite.  The  granular  permutite  is  freed  from  small 
particles,  and  the  reacting  solution  is  run  through  it  at  the 
rate  of  50  c.c.  per  hour.  In  this  way  complete  equilibrium 
is  obtained. 

Sodium  permutite  yields  the  corresponding  potassium 
permutite  with  potassium  chloride  and  potassium  sulphate, 
and  the  ammonium  permutite  is  also  completely  converted 
into  potassium  permutite.  The  total  concentration  of  the 
solution  does  not  affect  the  composition  of  the  final  product, 
nor  does  the  nature  of  the  alkali  in  the  original  permutite. 


OTHER  AUJMINOSIUCATES. 

Beryl  consists  of  silica,  alumina,  and  oxide  of  beryllium 
(or  glucinum,  as  it  is  sometimes  called),  and  has  the  formula 
Be3Al2Si6O18.  It  usually  occurs  in  long  hexagonal  prisms 
showing  vertical  striations.  The  colour  of  beryl  may  be 
blue,  green,  yellow,  brown,  or  rarely  pink,  but  it  is  sometimes 
colourless.  The  specific  gravity  is  about  2*7,  and  the  hard- 
ness 7^  to  8.  The  gem  varieties  are  transparent,  but 
coarse  varieties  of  beryl  may  be  opaque.  It  is  found  in 
several  parts  of  Ireland,  in  the  Grampians,  and  in  Cornwall, 
as  well  as  in  numerous  foreign  localities.  A  large  crystal 
found  in  the  United  States  weighed  more  than  2j  tons. 
By  weathering,  beryl  undergoes  alteration,  and  may  give 
rise  to  kaolin  and  mica. 

Aquamarine  is  a  variety  of  beryl  of  a  pale-green  or  sky- 
blue  colour,  and  is  made  into  various  articles  of  jewellery 
(brooches,  necklaces,  bracelets)  and  also  into  seals  and 
intaglios  (the  latter  having  the  engraved  parts  below  the 
general  level  of  the  surface,  as  compared  with  cameos,  in 
which  the  engraved  parts  stand  out  in  relief).  Weak  colour 
and  deficient  lustre  are  the  most  noticeable  defects  of  beryl, 
which  is  a  stone  easily  worked  without  risk  during  cutting 
and  polishing.  The  only  stone  for  which  the  aquamarine 


ioo  SILICA   AND   THE  SILICATES 

is  likely  to  be  mistaken  is  the  blue  topaz,  but  the  latter  is 
harder,  heavier,  and  more  lustrous. 

Emerald  is  a  bright  green  transparent  variety  of  beryl, 
the  other  characters  being  mostly  the  same.  The  colour  is 
probably  due  to  a  little  chromium  present.  The  stone 
losos  its  colour  when  strongly  heated. 

"  Oriental  emerald "  is  a  green  corundum  (consisting 
essentially  of  alumina,  like  the  ruby  and  sapphire)  ;  "  lithia 
emerald  "  is  the  rare  mineral  hiddenite  (a  variety  of  spodu- 
mene,  already  referred  to  among  the  pyroxenes) ;  "  Brazilian" 
emerald  is  green  tourmaline  ;  "  evening  emerald  "  is  the 
peridot ;  "  pyro-emerald  "  is  fhior  spar  which  phosphoresces 
with  a  green  glow  when  heated  ;  and  "  mother  of  emerald  " 
is  usually  a  green  quartz,  but  possibly  sometimes  a  green 
felspar. 

Euclase  is  a  rare  mineral  composed  of  silica,  alumina, 
beryllium  oxide,  and  water,  represented  by  the  formula 
HBeAlSiOg  or  Be(AlOH)SiO4,  or  2$iO2.Al2O3.2BeO.H2O. 
The  colour  is  generally  pale  blue  or  green,  but  sometimes 
colourless.  Its  hardness  is  7^,  and  its  specific  gravity  3-1. 
It  cleaves  readily  (whence  the  name),  which  makes  it  difficult 
to  work,  but  it  is  sometimes  cut  as  a  gem-stone.  The  cut 
stone  resembles  aquamarine  and  topaz,  but  its  specific 
gravity  serves  to  distinguish  it. 

Idocrase  or  Vesuvianite  consists  of  silica,  alumina,  and 
lime,  with  ferric  oxide  and  magnesia  partially  replacing 
alumina  and  lime  respectively.  The  hardness  is  6£,  and  the 
specific  gravity  about  3-4.  The  colour  is  mostly  brown  or 
green,  but  sometimes  yellow  or  blue,  and  rarely  black.  It 
is  generally  translucent,  and  sometimes  nearly  transparent. 
Italian  specimens  are  cut  as  gems  for  rings  and  other  orna- 
ments at  Naples  and  Turin,  and  often  sold  as  hyacinth, 
chrysolite,  etc.  A  compact  massive  variety  from  California 
called  californite  is  extensively  used  as  a  gem-stone,  and 
also  for  larger  ornaments.  This  variety  is  somewhat 
translucent,  takes  a  high  polish,  and  is  bright-green  to 
yellowish-green  and  white,  closely  resembling  jade  in 
appearance. 


SILICATES  101 

lolite,  Cordierite,  or  Dichroite  contains  silica,  alumina, 
magnesia,  and  ferrous  oxide.  Hardness  7  to  7^ ;  specific 
gravity  about  2 '6.  It  is  transparent  or  translucent,  usually 
of  a  pale  or  dark-blue  colour,  but  sometimes  colourless,  or 
grey,  or  brown,  etc.  It  occurs  at  Dalkey  (county  Dublin) 
and  Rathlin  Island,  Inverness-shire,  and  various  places 
abroad.  It  is  sometimes  used  as  an  ornamental  stone. 
Small  transparent  rolled  masses  of  a  blue  colour  (but  partly 
colourless)  give  the  Sapphire  d'eau  of  jewellers.  lolite  is 
remarkable  for  the  different  colours  it  displays  when  looked 
through  in  different  directions. 

Finite  is  an  alkaline  variety  of  altered  iolite,  in  which 
magnesia  and  ferrous  oxide  are  partly  replaced  by  potash. 
It  is  slightly  translucent  or  opaque,  with  hardness  about  2£, 
and  specific  gravity  about  2 '8.  It  occurs  in  Cornwall  and 
Aberdeenshire,  and  in  many  foreign  localities. 

Leucite,  the  composition  of  which  is  represented  by 
KAlvSi2O6,  or  4SiO2.Al2O3.K2O,  is  a  whitish  or  greyish 
volcanic  mineral.  Hardness  5j  to  6,  specific  gravity  about 
2*49.  It  has  been  proposed  as  a  source  of  potash,  being 
easily  decomposed  by  hydrochloric  acid,  like  nephelite. 
I^eucite  has  sometimes  been  termed  white  garnet. 


OTHER  SILICATES,  ETC. 

Glauconite  is  a  green  mineral,  a  hydrated  silicate  of 
iron  and  potassium.  It  occurs  in  green  sands  and  muds 
now  accumulating  at  many  places  on  the  sea  bottom.  The 
mud  and  sand  are  derived  from  the  wear  of  the  Continents, 
distributed  by  sea  currents.  The  material  consists  mainly 
of  quartz,  felspar,  mica,  chlorite,  and  calcite,  the  latter 
probably  derived  from  organisms.  The  colouring  matter 
is  believed  to  be  glauconite,  which  is  crystalline,  but  never 
occurs  well  crystallized,  but  only  as  clusters  of  extremely 
small  particles  which  slightly  affect  polarized  light.  They 
often  form  rounded  lumps,  which  in  many  cases  are  certainly 
internal  casts  of  empty  shells  of  Foraminifera.  The  mineral 
originated  among  the  sand  and  mud  on  the  sea  bottom,  and 


102  SILICA   AND   THE  SILICATES 

is  so  soft  and  friable  that  it  cannot  have  been  transported 
far  by  currents.  Small  rounded  lumps  of  glauconite,  common 
on  the  sands,  but  showing  no  indication  of  having  rilled  the 
shells  of  Foraminifera,  may  have  arisen  by  redeposition  of 
broken-down  casts.  The  green  sands,  when  weathered,  are 
brown  or  rusty  coloured,  the  glauconite  being  oxidized  to 
limonite. 

Glauconite  has  been  suggested  as  a  source  of  potash,  but 
British  glauconites  only  contain  i  to  4  per  cent.  It  may, 
however,  act  as  a  fertilizer  by  supplying  potash  to  the  soil. 

Zircon,  SiO2.ZrO2  or  ZrSiO4,  zirconium  orthosilicate, 
is  transparent  to  opaque,  and  is  mostly  red,  brown,  yellow, 
green,  or  grey,  but  rarely  white.  The  hardness  is  7j,  and 
specific  gravity  4  to  4*8  ;  it  is  transparent  to  opaque.  When 
strongly  heated  it  loses  its  colour  without  melting.  Trans- 
parent varieties  which  are  colourless  or  nearly  so  are  called 
jargon  or  jargoon,  and  red  transparent  varieties  constitute 
the  hyacinth  or  jacinth,  both  of  these  being  valuable  gems. 
The  colourless  varieties  are  sometimes  sold  for  diamonds, 
and  the  commercial  hyacinth  is  often  essonite,  a  variety  of 
garnet.  Zircon  is  often  used  in  jewelling  watches.  Zircons 
are  found  in  several  localities  in  Scotland,  and  in  Ireland, 
as  well  as  in  many  places  abroad. 

Sphene  or  Titanite,  SiO2.TiO2.CaO,  or  CaSiTiO5,  is  a 
silicate  and  titanate  of  lime.  It  is  transparent  to  opaque  ; 
and  brown,  grey,  yellow,  green,  or  black.  The  hardness  is 
5  to  5 1,  and  specific  gravity  about  3-5.  It  occurs  in  various 
localities  in  Scotland,  Ireland,  England,  and  Wales,  as  well 
as  abroad.  It  is  occasionally  used  as  an  ornamental  stone. 

GRANITE  AND  OTHER  IGNEOUS  ROCKS. 

By  the  term  igneous  is  implied  that  the  rock  is  or  was  at 
some  time  in  a  melted  condition,  and  according  to  the 
circumstances  attending  the  cooling  they  acquire  certain 
distinctive  characters  which  enable  them  to  be  classified. 

Granite  is  a  name  commonly  applied  to  almost  any 
kind  of  rock  which  is  crystalline,  and  is  like  true  granite  in 


SILICATES  103 

appearance  and  properties.  Strictly  the  name  should  be 
applied  only  to  rocks  which  are  completely  crystalline  in 
structure,  contain  a  relatively  large  proportion  of  silica 
(approaching  70  per  cent.,  or  over),  and  consist  essentially 
of  quartz  and  felspar,  generally  along  with  relatively  small 
amounts  of  mica,  hornblende,  augite  (rarely),  and  other 
minerals. 

True  granites  vary  much  in  colour  and  appearance 
according  to  the  condition  of  the  felspars  which  constitute 
their  most  abundant  minerals,  and  to  the  relative  proportions 
of  biotite,  hornblende,  and  other  dark-coloured  silicates. 
The  colour  ranges  from  white  or  grey  to  pink,  greenish,  or 
yellow.  Some  granites  contain  large  prominent  crystals  of 
felspar,  as  Shap  granite  and  some  Cornish  granites.  Other 
granites  are  more  or  less  fine  grained,  and  when  larger 
crystals  appear  distributed  through  a  fine-grained  mass,  the 
rock  becomes  a  porphyry. 

From  the  main  body  of  the  granite  rock,  veins  or  other 
branches  often  run  out  among  the  surrounding  rocks, 
showing  that  the  granite  has  forced  its  way  upwards  by 
splitting  the  strata  among  which  it  is  placed.  This  conclusion 
is  confirmed  by  the  effects  caused  by  the  forcible  passage 
of  highly-heated  material  through  the  surrounding  rocks. 
Thus  limestones  become  recrystallized  as  marbles.  As  the 
result  of  long  continued  weathering,  some  granites  form 
large  blocks  which  may  give  rise  to  structures  suggestive 
of  cyclopean  masonry,  as  in  the  tors  of  the  west  of  England. 
Such  differences  depend  on  the  arrangement  of  the  joint 
cracks  in  the  rock,  since  it  is  along  these  that  weathering 
action  proceeds. 

Most  granites  are  coarse  grained  enough  to  enable  the 
chief  mineral  constituents  to  be  distinguished  by  the  unaided 
eye.  The  felspar  appears  pearly,  usually  white  or  pink, 
with  smooth  flat  cleavage  surfaces  ;  quartz  is  generally 
transparent  and  glassy,  with  rough  and  irregular  fracture 
surfaces  ;  micas  form  light  or  dark  flakes.  Sometimes  small 
dark  green  or  black  prisms  (crystals)  of  hornblende  can  be 
detected,  and  sometimes  reddish  grains  of  garnet  or  of 


104  SILICA   AND   THE  SILICATES 

sphene.     In   tourmaline    granites   prisms    of   black    schorl 
occur. 

Very  coarse  granite  is  called  pegmatite,  a  variety  which 
commonly  contains  little  besides  felspar  and  quartz.  It  is 
much  used  on  the  Continent  as  an  ingredient  for  hard 
porcelain. 

Many  varieties  of  granite  have  received  distinctive 
names,  and  some  rocks  popularly  regarded  as  granites 
should  properly  be  referred  to  other  types.  This  applies 
especially  to  rocks  in  which  the  amount  of  quartz  is 
materially  reduced,  and  consequently  the  total  percentage 
of  silica  is  less. 

Thus  syenite  consists  essentially  of  hornblende  and 
orthoclase,  and  diorite  mainly  of  hornblende  and  plagioclase 
felspar,  often  with  some  dark  mica  (biotite)  ;  the  presence 
of  these  dark-coloured  minerals  makes  the  rock  itself 
darker  in  colour.  A  variety  of  syenite  containing  elseolite 
(a  form  of  nephelite)  is  called  elceolite  syenite. 

Uses  of  Granite. — As  building  stone,  for  monumental 
stones,  and  for  interior  decorations.  Also  for  curbing, 
paving,  etc.,  and  often  as  road  metal.  Also  as  ballast, 
rubble,  etc. 

Disintegration  of  Igneous  Rocks. — As  a  result  of 
weathering  and  other  influences,  the  minerals  of  rocks 
gradually  undergo  decomposition,  and  among  the  first  to 
suffer  such  changes  are  the  felspars.  The  decomposition 
of  rocks  containing  felspar  sometimes  results  in  the  pro- 
duction of  China  clay.  Partially  decomposed  granite  is 
extensively  used  in  the  ceramic  industry  under  the  name 
of  Cornish  stone  or  China  stone. 

Lava  is  the  name  given  to  liquid  products  of  volcanic 
activity.  Geologically  the  term  includes  all  matter  derived 
from  volcanoes,  which  flows  or  did  flow  in  a  molten  condition. 
A  lava  is  said  to  be  acid  when  high  in  silica,  and  basic  when 
comparatively  low  in  silica.  Basic  lavas  are  generally 
darker  and  specifically  heavier  than  acid  lavas,  as  well  as 
less  viscous. 

On  exposure  to  the  air,  lava  quickly  solidifies  on  the 


SILICATES  105 

surface  to  form  a  crust,  which  is  often  broken  up  by  the 
continued  flow  of  the  liquid  lava  below,  resulting  in  the 
formation  of  a  rugged  surface. 

Each  type  of  crystalline  igneous  rock  has  its  correspond- 
ing lava,  rhyolite,  trachyte,  and  andesite,  being  the  lavas 
corresponding  to  granite,  syenite,  and  diorite  respectively, 
whilst  basic  lavas  are  known  as  basalts. 

Basalt,  as  previously  indicated,  is  an  igneous  rock  of  a 
basic  type,  with  low  percentage  of  silica.  It  is  of  a  dark 
colour,  and  weathers  brown.  Many  basalts  are  very  fine 
grained  and  compact,  but  most  of  them  show  larger  crystals 
of  olivine  or  augite,  and  often  felspar  in  a  finely  crystalline 
ground  mass.  The  olivine  is  green  or  yellowish,  augite 
black,  and  felspar  dark  grey  with  smooth  cleavage  faces. 
Basaltic  lavas  are  often  spongy  or  pumiceous,  especially 
near  the  surface  ;  the  steam  cavities  in  course  of  time  become 
filled  with  secondary  minerals  (calcite,  chlorite,  zeolites,  etc.). 
Many  basaltic  rocks  show  columnar  jointing,  famous 
examples  of  which  occur  at  the  Giant's  Causeway.  The 
olivine  in  basalt  is  often  partly  altered  to  serpentine. 

In  many  basalts  small  crystals  of  felspar  and  augite 
form  a  matrix  including  more  or  less  of  a  brown  isotropic 
glass.  Olivine  is  rarely  an  ingredient  of  the  ground  mass. 

In  the  vitreous  basalts,  most  of  the  rock  is  a  dark  brown 
glassy  substance,  generally  containing  black  grains  of 
magnetite  with  skeleton  crystals  of  augite  or  felspar, 
spherulites,  perlitic  cracks,  or  steam  vesicles.  In  other 
basaltic  rocks  called  dolerites  there  is  no  glassy  material, 
the  whole  mass  being  thoroughly  crystallized. 

Among  other  minerals  sometimes  present  in  basaltic 
rocks  are  black  hornblende,  hypersthene  (usually  replacing 
olivine),  and  black  mica  (biotite),  nephelite,  leucite,  etc. 

Basalt  is  largely  used  in  the  construction  of  permanent 
roads. 

Volcanic  Tuff  or  Tufa  is  a  consolidated  mass  of  volcanic 
ashes,  composed  of  loose  fragments  ejected  from  volcanoes. 
It  is  occasionally  used  for  building  stone. 

Certain  volcanic  ashes  have  been  largely  employed  for 


106  SILICA   AND   THE  SILICATES 

making  hydraulic  mortars,   and  will  be  referred  to  more 
fully  in  the  section  on  "  L,ime,  Cement,  and  Mortar." 

Obsidian  or  Volcanic  Glass  is  a  glassy  volcanic  rock  of 
acid  composition,  containing  over  70  per  cent,  of  silica. 
Rhyolitic  lavas  often  tend  to  be  vitreous,  and  when  the  glassy 
material  predominates  to  such  an  extent  that  the  crystalline 
portions  are  insignificant,  the  lava  becomes  an  obsidian  on 
cooling.  The  chemical  composition  is  essentially  that  of 
granite,  and  the  difference  in  appearance,  etc.,  is  due  to  the 
conditions  which  prevailed  during  cooling.  In  the  case  of 
the  obsidian  solidification  took  place  rapidly  at  the  earth's 
surface,  and  under  low  pressure,  whereas  granite  cooled 
very  slowly  at  a  considerable  depth  below  the  surface  and 
under  great  pressure.  Most  obsidians  have  small  crystals 
of  felspar,  quartz,  biotite,  or  iron  oxides. 

The  specific  gravity  of  obsidian  is  low,  about  2*4.  In 
thin  splinters  or  on  sharp  edges  they  are  transparent,  and 
the  prevailing  colours  are  black,  grey,  yellow,  and  brown. 
Obsidian  often  shows  a  banded  structure,  indicating  want  of 
homogeneity  of  the  molten  material. 

Even  when  crystals  seem  to  be  absent,  minute  imperfect 
crystallizations  called  microlites  are  nearly  always  very 
numerous,  as  well  as  formations  suggestive  of  skeleton 
crystals  known  as  crystallites,  and  also  spherulites,  or  small 
rounded  (generally  globular)  bodies  having  a  radiating 
fibrous  structure.  Sometimes  contraction  is  associated 
with  the  formation  of  minute  cracks,  occasionally  concentri- 
cally surrounding  little  globules  of  glass  ;  in  some  cases  the 
minute  globules  have  a  somewhat  pearly  lustre,  and  this 
kind  of  structure  is  termed  perlitic. 

Uses  of  Obsidian. — Arrow-heads,  spear-heads,  knives, 
mirrors,  etc.,  of  obsidian  were  much  used  by  the  ancient 
Mexicans  and  other  primitive  races.  The  ancient  Greeks  and 
Romans  worked  obsidian  as  a  gem-stone.  It  is  still  some- 
times cut  and  polished  as  an  ornamental  stone. 

Similar  glassy  material  of  basic  composition  occurs, 
closely  resembling  obsidian,  but  it  is  rather  rare. 

When  a  glassy  volcanic  rock  has  a  rather  dull  resinous 


SILICATES  107 

(instead  of  bright  vitreous)  lustre  it  is  termed  pitchstone. 
Pitchstone  contains  5  to  10  per  cent,  of  combined  water,  and 
nearly  always  contains  numerous  minute  crystallites  and 
microlites. 

Pumice  is  a  very  porous,  froth-like  volcanic  glass.  It 
cooled  too  quickly  to  be  able  to  crystallize,  and  the  sudden 
release  of  the  dissolved  vapours  when  it  solidified  caused  it 
to  swell  up  into  a  froth  which  at  once  became  solid.  If  it 
had  cooled  under  greater  pressure  an  obsidian  or  volcanic 
glass  would  have  been  formed.  Indeed,  fragments  of 
obsidian  heated  until  they  melt  will  suddenly  change  to 
pumice  owing  to  escape  of  dissolved  gases.  Similar  material 
can  be  formed  artificially  by  blowing  steam  through  molten 
slag  or  glass.  When  slag  is  cooled  very  suddenly  by  being 
tipped  into  the  sea — as  happens  at  the  Whitehaven  blast 
furnaces — it  swells  up  into  a  pumice-like  material  so  full  of 
bubbles  and  so  light  that  it  will  actually  float  on  water. 
Pumice  is  often  found  as  a  layer  above  obsidian. 

Uses  of  Pumice. — An  inferior  pumice  is  used  for 
smoothing  oil-cloth.  Much  pumice  is  used  in  fine  powder 
(produced  by  crushing  the  rock)  to  form  an  ingredient  of 
metal  polishes  and  certain  soaps.  Pumice  has  also  been 
used  for  polishing  stones,  glass,  and  ivory,  and  in  a  powdered 
state  (called  pounce)  for  preparing  parchment.  Powdered 
pumice  has  also  been  used  for  making  bricks  of  very  low 
specific  gravity  and  good  insulating  quality. 

Gneiss  is  a  metamorphic  rock,  having  undergone 
important  changes  from  a  previous  condition.  It  may 
originate  (like  some  other  metamorphic  rocks)  from  pre- 
existing sedimentary  or  igneous  rocks.  It  consists  mainly 
of  quartz  and  felspar,  with  some  mica,  hornblende,  or 
augite,  and  other  minerals  of  less  importance.  All  gneisses 
have  a  foliated  (or  parallel)  structure — particularly  as 
regards  the  mica  or  hornblende,  etc. — and  this  is  generally 
the  chief  distinction  between  gneiss  and  granite,  as  they 
have  much  the  same  mineralogical  composition. 

The  felspars  of  gneiss  are  chiefly  orthoclase  and  oligoclase. 
Muscovite  and  biotite  may  occur  separately  or  together. 


io8  SILICA   AND  THE  SILICATES 

Many  gneisses  have  been  called  by  special  names  based 
on  a  prominent  mineral  component,  as  mica-gneiss,  horn- 
blende-gneiss, etc. 

A  schist  consists  of  minerals  arranged  in  alternating 
layers,  the  foliation  being  more  distinct  usually  than  in 
gneiss.  The  most  common  schist  is  mica  schist,  consisting 
essentially  of  quartz  and  mica.  Among  other  schists 
occurring  are  chlorite  schist,  hornblende  schist,  etc. 

The  foliated  structure  renders  schists  and  gneiss  less 
suited  than  granitic  rocks  for  constructional  purposes  or 
paving,  etc. 

Shale  is  mud  or  clay  consolidated  by  pressure  (therefore 
fine  grained)  and  with  a  laminated  structure.  Consolidated 
silicious  material  may  be  similarly  formed  from  finely 
divided  sand-grains,  and  a  similar  shaly  rock  from  minute 
grains  of  calcite.  In  either  case,  if  clayey  matter  be  also 
present,  the  resulting  rock  is  best  classed  as  an  argillaceous 
sandstone  or  argillaceous  limestone  respectively. 

The  cohesion  of  shales  results  mainly  from  the  pressure 
exerted  during  consolidation.  Unlike  sandstones  they  have 
little  porosity.  Their  cleavage  is  very  imperfect. 

Some  shales  are  used  for  making  cement,  and  some  for 
making  bricks.  Bituminous  shales  yield  many  mineral 
oils. 

Slate  is  a  metamorphosed  clay  or  shale,  and  possesses 
a  good  cleavage,  resulting  from  considerable  pressure. 
This  cleavage,  known  as  slaty  cleavage,  does  not  often 
coincide  with  the  original  lamination  (which  corresponded 
with  the  stratification,  when  the  slate  was  not  of  igneous 
origin).  Most  slates  are  of  sedimentary  origin,  and  their 
cleavage  was  the  result  of  heavy  pressure,  or  of  lighter 
pressure  long  continued.  In  a  few  cases,  slates  have  been 
formed  by  the  consolidation  of  volcanic  ashes,  like  the  green 
slates  of  the  L,ake  district. 

In  quarrying  slates,  ordinary  blasting  powder  is  generally 
used,  but  experience  is  necessary  in  order  to  separate  large 
blocks  without  shattering  the  slate. 

Channelling  machines  are  often  used.     In  France  the 


SILICATES  109 

method  of  tunnelling  was  introduced  not  long  ago,  and 
subject  to  frequent  inspection  of  roof  and  walls,  this  marks 
a  distinct  advance. 

Manufacture  of  Roofing  Slates.— From  the  quarry 
large  blocks  of  slate  are  taken  to  the  blockmaker,  who  with 
the  aid  of  a  chisel,  hammer,  and  gouge,  divides  the  large 
blocks  into  slabs  about  2  ft.  by  i  J  ft.  by  2  ins.  thick.  These 
slabs  are  passed  to  the  splitter,  who  by  means  of  a  long, 
broad-edged  chisel,  and  a  maul,  reduces  the  slabs  to  a 
thickness  of  J  to  J  in.  Finally,  the  dresser,  or  trimmer, 
trims  all  the  slates  with  a  long  knife  set  vertically  and 
hinged  at  one  end,  the  other  end  being  raised  and  lowered  in 
turn,  either  by  hand  or  by  a  treadle.  For  splitting,  hand 
labour  is  still  used,  but  at  least  one  edge  of  the  blocks  is 
often  sawn  before  passing  them  to  the  splitter. 

The  slates  are  made  of  various  sizes. 

Uses  of  Slate. — Some  slates  are  adapted  for  blackboards, 
slates,  slate  pencils,  billiard  tables,  mantels,  and  urinals. 
Others  make  good  roofing  slates. 

Slates  are  also  used  for  underpinnings  of  buildings,  for 
floor  tiles,  risers  and  treads  of  stairs,  and  for  flags.  Slates 
may  sometimes  be  used  for  inexpensive  tablets  and  grave 
stones.  The  high  transverse  strength  of  thick  slabs  of 
slate  fits  them  as  substitutes  for  concrete  beams  in  many 
places. 

SI.AGS. 

Silica  plays  a  very  important  part  in  many  metallurgical 
operations  in  connection  with  the  formation  of  Slags,  which 
with  rare  exceptions  are  silicates,  and  are  always  anhydrous. 
These  slags  are  generally  the  means  for  removing  impurities 
from  the  ores. 

Silica  readily  combines  with  most  metallic  oxides,  when 
an  intimate  mixture  of  silica  and  the  oxide  is  heated  to  the 
proper  temperature,  if  the  oxide  itself  is  not  reducible  up  to 
the  temperature  necessary  for  combination.  Fusion  may 
not  be  necessary  for  combination.  When  certain  mixtures 
of  silica  and  li  ne  are  strongly  heated,  there  is  no  indication 


no  SILICA   AND  THE  SILICATES 

of  fusion,  but  the  action  of  hydrochloric  acid  on  the  product 
causes  gelatinous  silica  to  separate,  which  is  evidence  of  the 
silica  having  been  in  combination. 

The  bases  most  frequently  present  in  slags  are  lime, 
magnesia,  ferrous  oxide,  manganese  oxide,  alumina,  and 
potash  in  small  amount.  The  bases  and  silica  are  derived 
from  the  ore,  fuel  ash,  added  flux,  and  to  some  extent  from 
the  furnace  materials.  It  is  only  in  blast  furnaces  that  the 
ashes  of  the  fuel  pass  into  the  slag. 

Slags  may  consist  of  a  single  silicate,  of  a  combination 
of  silicates,  or  of  a  mixture  of  silicates,  with  or  without 
foreign  matter.  Slags  may  be  distinctly  crystallized,  and 
yet  not  have  a  perfectly  definite  constitution,  just  like  many 
natural  minerals.  A  South  Staffordshire  blast  furnace 
slag  occurring  in  translucent  square  prisms  gave  the  following 
results  on  analysis  : — 

38*05  silica  0*40  manganese  oxide 

14-11  alumina  1/27  ferrous  oxide 

3570  lime  1-85  potash 

7 '60  magnesia  0*82  calcium  sulphide. 

This  nearly  corresponds  to  9SiO2.2Al2O3.i2RO  (where  RO 
includes  all  the  oxides  other  than  alumina  and  silica). 

Slags  occur  crystallized,  glassy,  or  non-crystalline  and 
stone-like,  and  all  these  characters  may  appear  in  a  single 
piece,  in  which  case  the  glassy  portion  (produced  by  rapid 
cooling)  will  be  on  the  outside.  Slags  bear  a  close  resem- 
blance to  volcanic  lavas.  The  more  vitreous  slags  also 
resemble  glass. 

Slags  may  be  more  or  less  porous  or  vesicular.  Occasion- 
ally slag  from  blast  furnaces  has  been  obtained  in  the  form 
of  spun  glass,  caught  by  the  blast  and  blown  into  fine 
threads,  exactly  like  the  Pele's  hair  or  capillary  volcanic 
glass  from  Hawaii. 

A  slag  is  generally  tougher  the  more  slowly  it  cools, 
just  as  devitrified  glass  (likewise  resulting  from  slow  cooling) 
is  very  tough  in  comparison  with  the  original  glass.  Some 
slags  are  very  brittle  both  in  the  glassy  and  crystallized 


SILICATES  in 

state.  When  slags  are  required  powdered  it  is  evidently 
an  advantage  to  cool  them  very  quickly  by  letting  them 
run  into  water. 

Slags  are  ordinarily  grey,  blue,  green,  red,  brown,  or 
black,  sometimes  veined  or  marbled  with  different  shades. 
The  colour  of  a  slag  sometimes  changes  on  crystallization, 
as  in  a  blast-furnace  slag  which  is  blue  when  vitreous  and 
yellowish-brown  when  crystallized,  both  colorations  often 
appearing  in  the  same  specimen.  So  in  bottle-glass  works 
specimens  may  be  found  of  brown  crystallized  nodular 
masses  embedded  in  a  blue  glassy  matrix.  The  blue  colour 
of  many  slags  was  attributed  by  Kersten  to  an  oxide  of 
titanium.  Fournet  concluded  that  the  blue  coloration  of 
slags  and  bottle-glass  was  due  simply  to  a  change  in 
molecular  arrangement.  The  blue  colour  of  slags  has  also 
been  doubtfully  referred  to  vanadium  and  artificial  ultra- 
marine. A  small  amount  of  sulphide  gives  slag  a  very 
dark  colour. 

Some  slags  when  melted  are  as  liquid  as  water,  but 
others  are  more  or  less  viscous. 

Scoriae  include  the  lighter,  more  porous,  and  imperfectly 
vitrified  materials  formed  mainly  during  the  puddling  and 
refining  of  iron,  and  resembling  volcanic  scoriae. 

Powdered  slags  and  scoriae,  when  mixed  with  hydraulic 
limes,  make  good  mortars.  Powdered  slag  alone  simply 
acts  like  sand,  but  certain  varieties  of  slag  when  granulated 
acquire  hydraulic  properties,  and  are  used  for  making  slag 
cements. 

Basic  slags,  when  ground,  make  good  fertilizers  for 
soil,  owing  to  the  calcium  phosphate  they  contain.  They 
have  been  extensively  used  for  this  purpose. 

Many  years  ago,  a  paper  by  Bashley  Britten  was 
prepared  for  a  meeting  of  the  Iron  and  Steel  Institute  at 
I«eeds  (see  the  Institute  Journal  for  1876,  p.  453),  in  which 
he  strongly  advocated  the  application  of  blast  furnace  slags 
in  the  fresh  molten  condition,  simply  mixed  with  common 
ferruginous  sand  and  sodium  sulphate,  for  the  manufacture 
of  bottle-glass.  Numerous  specimens  of  such  glass  exhibited 


H2  SILICA   AND   THE  SILICATES 

were  described  as  being  little  if  any  darker  than  much 
of  the  glass  used  in  the  form  of  rough  plates  for  skylights, 
and  of  practically  the  same  tint  as  the  glass  in  the  great 
conservatories  at  Kew  Gardens.  The  process  was  patented 
in  1876.  The  suggested  process  would  be  very  economical 
not  only  as  regards  materials,  but  also  by  making  use  of  the 
heat  in  the  molten  slag  now  commonly  wasted.  This  is  a 
problem  well  worth  serious  consideration  at  the  present 
time. 


FERROSIUCON. 

Ferrosilicon  or  iron  silicide  (sometimes  also  termed  ferro- 
silicide)  is  next  to  ferro-manganese  as  regards  the  amount 
used  in  the  metallurgy  of  iron  and  steel.  Ferrosilicons 
are  used  as  deoxidizers  or  for  making  silicon  steels,  and 
contain  from  8  per  cent,  to  nearly  95  per  cent,  silicon,  the 
latter  being  produced  in  electric  furnaces,  and  the  former 
in  English  blast  furnaces. 

The  structure  of  ferrosilicon  is  finely  or  coarsely  crystal- 
line according  to  the  proportion  of  silicon ;  in  high  grade 
alloys  with  more  than  50  per  cent,  of  silicon  there  is  no 
distinct  crystallization,  the  fracture  being  fine  and  bluish. 
At  least  three  iron-silicon  compounds  exist,  Fe2Si,  FeSi, 
and  FeSi2 ;  indications  of  the  existence  of  Fe2Si3,  Fe3Si2, 
and  FeSi4,  have  also  been  observed. 

Ferrosilicon  is  made  in  the  blast  furnace  or  the  electric 
furnace  :  (i)  by  reducing  silica  and  iron  ore  with  carbon, 
and  (2)  by  reduction  of  silica  with  carbon,  iron  turnings 
being  added.  In  the  blast  furnace,  fuel  consumption  is 
high  even  in  making  15  per  cent,  ferrosilicon,  and  the 
silicon  in  the  ordinary  grades  ranges  from  8  to  14  per 
cent.  Iron  oxide,  silicious  iron  ore,  and  coke  are  used 
in  blast-furnace  charges,  and  the  most  suitable  slags  are 
high  in  alumina  and  low  in  lime.  According  to  J.  E. 
Johnson's  U.S.  pat.  1,231,260  (June  26,  1917)  ferrosilicon 
containing  about  50  per  cent,  silicon  is  produced  by  charging 
the  blast  furnace  with  ore  and  fuel,  and  supplying  a  blast 


SILICATES  113 

containing  substantially  equal  parts  by  weight  of  oxygen 
and  nitrogen. 

In  the  electric  furnace,  quartz  or  sand,  and  charcoal, 
coke,  or  coal  are  used,  with  either  wrought  iron,  steel,  or 
cast-iron  turnings,  or  silicious  iron  ore.  When  turnings  are 
used  instead  of  iron  ore,  the  power  consumption  is  less, 
the  operation  of  the  furnace  is  steadier,  and  less  slag  is 
formed.  Most  of  the  phosphorus  and  sulphur  appears  in 
the  metal,  so  the  materials  should  be  pure. 

The  liquid  ferrosilicon  is  highly  corrosive  towards  iron, 
and  is  therefore  tapped  out  into  sand  or  carbon-lined  cars 
or  sand  moulds.  The  electric  furnace  product  is  in  four 
grades,  containing  respectively  25  to  30  per  cent.,  45  to  55, 
75  to  80,  and  90  to  95  per  cent,  silicon.  Alloys  with  30  to 
65  per  cent,  silicon  are  specially  liable  to  disintegrate, 
hydrogen  phosphide  and  arsenide  being  evolved,  which 
makes  them  dangerous  to  handle.  Alloys  with  less  than 
30  and  more  than  65  per  cent,  silicon  are  less  liable  to 
spontaneous  disintegration. 

Ferrosilicon  is  notable  for  the  fact  that  carbon  is  excluded 
from  the  iron  by  increase  of  the  silicon  from  about  4  per 
cent.  From  this  point  the  carbon  decreases  to  the  stage 
in  which  the  flakes  of  graphite,  previously  uninterrupted  in 
the  fracture  of  the  iron,  are  separated  by  white  spaces,  and 
this  action  continues  with  increase  of  the  silicon  until 
eventually  the  graphite  forms  isolated  black  stars  in  a 
white  field. 

The  lowering  of  the  carbon,  with  increase  of  silicon, 
imparts  to  the  iron  a  somewhat  greasy  look,  and  irons  with 
4  to  8  per  cent,  silicon  are  called  "  silvery  irons/'  because  of 
their  abnormally  light  colour.  These  irons  are  made  to 
some  extent  for  use  in  foundries,  to  increase  the  silicon  in 
mixtures  in  which  it  is  low. 

Silicon  promotes  soundness  in  ingot  iron,  but  should 
not  exceed  0*15  per  cent,  in  metal  which  has  to  be  rolled. 
About  0-30  per  cent,  silicon  promotes  soundness  in  steel 
castings.  Carbon  is  probably  detrimental  to  silicon  steel. 
Usually  the  25  to  30  per  cent,  ferrosilicon  is  used  for  making 
c.  8 


H4  SILICA   AND  THE  SILICATES 

definite  additions  of  silicon  to  the  metal,  and  the  45  to  50  per 
cent,  grade  for  deoxidation. 

In  foundry  practice,  if  special  strength  is  required,  the 
silicon  should  not  exceed  about  2  per  cent.,  and  the  phos- 
phorus should  be  rather  less  than  i  per  cent.  But  if  softness 
and  fluidity  are  specially  desired,  nearly  twice  these  amounts 
may  often  be  present  without  serious  injury.  Silicon  steel 
sheets  are  used  in  electric  dynamos  and  motors,  for  which 
their  magnetic  properties  make  them  fit. 

In  Iy.  Treuheit's  Ger.  pat.  292,682  (July  27,  1915) 
complete  deoxidation  of  iron  and  steel  alloys  is  effected  by 
placing  a  mixture  of  equal  weights  of  cryolite  and  ferro- 
silicon  in  the  ladle  and  pouring  the  steel  on  it.  The  forma- 
tion of  blowholes  and  slag  enclosures  is  said  to  be  almost 
entirely  eliminated. 

According  to  N.  G.  Petinot's  U.S.  pat.  1,274,360  (July 
30,  1918)  molten  ferrosilicon,  containing  10  to  16  per  cent, 
silicon,  is  poured  into  a  shaped  mould — first  into  a  mixer  if 
necessary — to  make  shaped  castings. 

For  particular  purposes  certain  special  alloys  are  some- 
times used. 

Ferrosilicon-aluminium  has  about  45  per  cent, 
silicon,  12  to  15  per  cent,  aluminium,  the  rest  being  iron. 
It  is  used  as  a  deoxidizer  for  steel  made  in  the  electric 
furnace.  It  is  more  effective  than  aluminium,  and  is  usually 
put  in  the  ladle  before  teeming. 

Ferrosilico-manganese  is  of  two  grades  :  60  to  70  per 
cent,  manganese,  20  to  25  silicon,  and  3  to  4  iron ;  and  50 
to  60  per  cent,  manganese,  22  to  25  silicon,  and  about  19 
iron.  In  both  the  carbon  is  about  0*35  per  cent.,  and 
sulphur  and  phosphorus  low.  These  alloys  are  sometimes 
used  for  adding  silicon  and  manganese  to  steel. 

Ferrosilico-manganese-aluminium  is  made  in  two 
grades,  containing  respectively  1 8  to  20  or  9  to  n  silicon, 
18  to  22  or  9  to  ii  manganese,  and  9  to  12  or  4-5  to  6  per 
cent,  aluminium. 

Ferrosilico-manganese-calcium-aluminium  has  an 
average  composition  of  50  to  55  per  cent,  manganese,  18  to 


SILICATES  115 

22  calcium,  12  to  15  iron,  4  to  5  aluminium,  and  about 
i  per  cent,  carbon.  This  alloy  is  a  deoxidizer  and  desul- 
phurizer,  used  only  with  high-grade  steels. 


SILICON  CARBIDE  (CARBORUNDUM,  ETC.). 

Silicon  Carbide  (SiC)  occurs  in  commerce  in  different 
forms. 

Carborundum,  a  crystalline  silicon  carbide,  was  dis- 
covered accidentally  by  E.  G.  Acheson  in  1891. 

Amorphous  Silicon  Carbide  is  obtained  as  a  greenish 
powder  by  heating  a  mixture  of  carbon  and  silica  at  about 
1200°,  but  carborundum  is  only  formed  at  a  much  higher 
temperature. 

At  Niagara,  carborundum  is  made  in  electric  furnaces 
built  of  firebricks  (E.  G.  Acheson's  Eng.  pat.  17911,  Oct.  7, 
1892).  The  end  walls  (carrying  the  terminals,  each  consist- 
ing of  30  carbon  rods  3  ins.  in  diameter)  and  the  bed  are  per- 
manent ;  the  sides  are  built  up  with  the  charge,  and  taken 
down  to  get  the  product.  The  charge  consists  of  34-2  parts 
coke,  54-2  sand,  9-9  saw-dust,  and  17  salt,  the  last  named 
serving  as  a  flux.  The  saw-dust  increases  the  porosity,  and 
so  facilitates  escape  of  the  carbon  monoxide  formed — 

SiO2-h3C  =SiC+2CO. 

About  six  tons  of  the  gas  escapes  during  the  run,  and 
burns  at  the  top  of  the  furnace.  The  furnace  is  half  filled 
with  the  mixed  materials,  so  as  not  to  touch  the  electrodes. 
A  cylindrical  core  21  ins.  diameter,  composed  of  coke  frag- 
ments J  to  f  in.  diameter,  is  then  built  up  between  the 
electrodes.  Over  the  core  the  mixed  materials  are  carried 
to  a  height  of  8  ft.  The  current  passing  through  the  coke 
forms  many  arcs  which  strongly  heat  the  charge. 

The  alternating  current  at  2200  volts  is  transformed 
down  to  165  volts,  and  the  heavy  current  is  regulated  or 
interrupted  by  means  of  a  large  water  rheostat.  Within 
i  J  hour  of  the  start  the  initial  E.M.F.  of  165  volts  is  reduced 
(owing  to  diminished  resistance)  to  125  volts,  and  the  current 


n6  SILICA   AND  THE  SILICATES 

increases  from  1700  to  6000  amperes,  after  which  the 
conditions  remain  steady  during  the  entire  run  of  36  hours. 
In  Tone's  modified  process  (U.S.  pat.  908,357,  1908), 
the  charge  is  preheated  by  ordinary  combustion  (of  gas  or 
solid  fuel).  The  process  can  be  made  continuous  (U.S.  pat. 
937,119,  1909)  by  changing  the  position  of  the  arc  relatively 
to  the  charge. 

The  charge  per  furnace  is  about  30,000  lb.,  the  core  being 
about  3  per  cent,  of  it.  The  electrical  energy  used  is  about 
26,400  kilowatt-hours,  and  the  product  contains  about 
6700  lb.  of  carborundum,  and  5000  lb.  of  amorphous  silicon 
carbide. 

The  carborundum  is  dug  out  and  transferred  to  a 
mechanical  crusher  with  water,  then  digested  for  three  days 
at  100°  C.  with  a  mixture  of  sulphuric  acid  and  2  parts  of 
water,  and  afterwards  washed  with  water.  The  finer 
material  which  is  washed  away  is  collected  by  settling  for 
definite  periods,  and  forms  carborundum  flours.  The 
remaining  material  is  dried  in  a  kiln,  then  graded  by  means 
of  screens. 

In  Weber's  method  (U.S.  pat.  728,528,  1903)  a  mixture  of 
kaolin  and  coke  is  heated  in  an  electric  furnace.  The 
product  is  treated  with  water,  which  decomposes  the 
aluminium  carbide,  leaving  the  carborundum. 

By  the  action  of  vapours  of  carbon  and  silicon  upon 
silicon  carbide,  a  dense  compact  variety  is  produced,  which 
is  very  hard,  resistant  to  acids,  and  a  good  insulator  (U.S. 
pat.  913,324)- 

In  Potter  and  Westinghouse's  method  (U.S.  pat.  875,673, 
1907)  a  mixture  of  silicon  monoxide  and  carbon  is  used. 

Articles  shaped  in  carbon  may  be  converted  into  carbo- 
rundum by  very  strongly  heating  in  a  bed  of  very  finely 
powdered  carborundum  or  of  sand  and  carbon  (Boiling, 
Eng.  pat.  6693,  1905). 

Commercial  carborundum  contains  6270  per  cent, 
silicon,  32*26  carbon,  0*93  alumina  and  ferric  oxide,  ando'ii 
magnesia  ;  after  purification  by  heating  in  oxygen  for  one 
hour  and  treating  with  hot  sulphuric  and  hydrofluoric  acids, 


SILICATES  117 

the  composition  was  69*10  silicon,  30*20  carbon,  0*46  alumina 
and  ferric  oxide,  and  0*15  lime.  Analysis  of  amorphous 
silicon  carbide  gave  65*42  silicon,  27*93  carbon,  5*09  alumina 
and  ferric  oxide,  0*38  lime,  and  0*21  magnesia.  For  pure 
silicon  carbide  the  theoretical  figures  are  70*00  silicon  and 
30*00  carbon. 

Crystalline  products  resembling  carborundum,  bearing 
the  trade  names  cry  stolon,  carbolon,  etc.,  are  manufactured 
by  other  American  firms.  In  addition  to  the  United  States, 
silicon  carbide  is  also  manufactured  to  a  limited  extent  in 
Canada  and  in  France. 

Silfrax  is  another  similar  material  suitable  for  electrical 
resistance  material,  as  a  refractory,  and  for  chemical  utensils. 
It  has  about  91  to  93  per  cent,  of  silicon  carbide,  the  rest 
being  silica,  carbon,  iron,  and  aluminium. 

Properties. — Carborundum  takes  the  form  of  flattened 
hexagonal  rhombohedra,  with  hardness  9^  (inferior  only  to 
the  diamond),  and  specific  gravity  3*23.  Commercial 
carborundum  is  greenish-grey  to  yellow  or  blue,  but  when 
prepared  in  a  pure  state  from  pure  materials  it  is  colourless. 

Carborundum  is  infusible,  and  not  affected  by  heat  below 
2220°,  at  which  temperature  it  is  decomposed  into  silicon  and 
graphitic  carbon.  Acids  do  not  affect  it,  but  fused  alkalies 
attack  it,  forming  a  silicate  and  carbon. 

Uses. — The  chief  use  of  carborundum  is  as  an  abrasive 
to  replace  emery  ;  it  is  more  expensive,  but  acts  more 
effectively  in  one-third  to  one-fourth  the  time.  Silicon 
carbide  is  the  most  satisfactory  abrasive  for  brass,  bronze, 
granite,  marble,  leather,  wood,  and  other  materials  of  lower 
tensile  strength  than  malleable  iron,  but  for  steels  in  general, 
and  high-speed  steels  in  particular,  an  aluminous  abrasive 
is  better,  in  spite  of  its  inferior  hardness,  because  of  its 
greater  toughness.  The  powder  of  carborundum,  etc.,  is 
used  in  glass  cutting  and  grinding,  and  for  polishing. 

For  making  wheels,  hones,  and  other  useful  objects, 
carborundum  is  mixed  with  moistened  kaolin  and  felspar, 
moulded  by  hydraulic  pressure,  and  fired  in  a  kiln  for 
7  days.  For  special  purposes  other  binding  materials 


n8  SILICA   AND  THE  SILICATES 

are  used  (as  shellac).  It  is  also  used  for  making  papers  and 
cloths  in  the  same  way  as  emery  and  garnet  powders. 
Binding  material  may  be  omitted  if  the  carborundum  be 
moulded  with  water,  and  the  articles  heated  to  2500°  F.  in 
an  oxidizing  flame. 

For  drill  heads,  carborundum  can  be  substituted  for 
diamond,  if  fixed  in  a  suitable  metallic  or  ceramic  matrix. 

Silicon  carbide  is  sometimes  used  instead  of  ferrosilicon 
as  a  source  of  silicon  in  the  manufacture  of  steel.  About 
O'i  to  0-4  per  cent,  is  placed  in  the  ladle,  and  dissolves 
readily  in  the  molten  steel,  ensuring  the  production  of 
solid  castings. 

Amorphous  silicon  carbide,  formerly  a  waste  material, 
is  used  for  making  highly  refractory  bricks,  and  the  retorts 
for  distillation  of  zinc.  It  is  good  for  lining  electric-resistance 
furnaces  or  fuel-fired  furnaces,  in  the  latter  even  as  a  wash 
for  firebrick  walls.  It  is  also  useful  as  a  cement  for  fire- 
bricks. 

Powdered  crystalline  silicon  carbide,  moulded  with 
glue  or  dextrin,  etc.,  and  heated  in  an  electric  furnace, 
gives  strong  bricks  good  for  the  roofs  of  electric  steel 
furnaces ;  they  must,  however,  be  backed  with  firebricks 
or  other  heat-insulating  material. 

A  refractory  cement  is  made  from  90  to  60  parts  carbo- 
rundum, 10  to  40  fireclay,  o  to  4  lime,  and  20  to  50  water- 
glass  solution  (47  Be.),  the  whole  being  mixed,  dried,  and 
powdered  again. 

By  the  action  of  silicon  carbide  on  metallic  oxides,  many 
metallic  silicides  may  be  obtained.  This  reaction  may  be 
used  for  the  preparation  of  special  ternary  or  quaternary 
steels  at  a  single  operation. 

As  far  back  as  1898  (Eng.  pat.  24,378)  Engels  applied 
mixtures  of  silica  and  carbon  to  surfaces  of  bricks,  and  to 
walls  of  furnaces,  etc.,  and  fused  them  by  the  electric  arc 
to  form  fireproof  and  acid-proof  coatings  of  silicon  carbide. 

Several  related  substances  contain  oxygen  as  well  as 
silicon  and  carbon. 

Siloxicon  is  produced  by  strongly  heating  a  mixture  of 


SILICATES  119 

graphite  and  silica  in  an  electric  furnace.  The  typical 
formula  is  Si2C2O.  When  pressed  in  a  mould,  and  heated 
to  2500°  F.  it  is  found  to  be  self-bonding.  It  is  infusible, 
but  at  very  high  temperatures  is  converted  into  carborundum. 

Silundum,  Si4C4O,  is  another  electric  furnace  product, 
obtained  by  the  action  of  silicon  vapour  or  a  mixture  of 
silicon  vapour  and  carbon  monoxide  on  carbon  at  temper- 
atures above  1300°  to  about  1800°  C.  Above  1800°  it  appears 
to  be  converted  into  carborundum,  and  above  2200°  C. 
graphite  is  formed.  Silundum  has  hardness  9,  and  specific 
gravity  2*9  to  3. 

Fibrox  (U.S.  pat.  1094352,  1914 ;  Eng.  pat.  16299, 
July  15,  1913)  is  a  silicon  oxycarbide  (approximating  to 
SiCO),  prepared  by  melting  silicon  with  a  catalytic  agent,  as 
calcium  fluoride,  in  a  muffle  or  crucible,  carbon  monoxide  and 
carbon  dioxide  diffusing  slowly  through  the  containing  vessel, 
and  combining  with  the  silicon  to  form  a  soft  resilient  fibrous 
material.  The  specific  gravity  of  the  fibres  is  i'8  to  2 '2, 
and  the  apparent  specific  gravity  is  0*0025  to  0*0030  (more 
than  99  per  cent,  of  the  space  being  occupied  by  air).  It  is 
a  very  good  heat  insulator. 


GEM  CUTTING  AND  LAPIDARY'S  WORK. 

Gem  carving  by  the  ancients  previous  to  the  third  or 
fourth  century  A.D.  seems  to  have  been  done  by  hand  with 
a  sapphire  point  (fixed  in  a  handle),  or  with  a  bow  drill,  but 
from  that  time  the  lapidary's  wheel  came  into  extensive 
use.  In  the  fifth  and  sixth  centuries  B.C.,  and  for  two  or 
three  hundred  years  later,  the  Greek  cutters  preferred  the 
sapphire  point  for  their  finest  work,  though  they  knew  how 
to  use  discs  and  drills.  The  bow  drill  was  a  similar  point 
fixed  at  the  end  of  a  stick,  which  could  be  rotated  (alternately 
in  opposite  directions)  by  the  up-and-down  movement  of  a 
horizontal  cross-bar  attached  at  each  end  to  a  string  wound 
around  the  stick.  The  two  appliances  were  afterwards 
worked  in  combination.  L,ater  a  hollow  tube  or  drill  was 
substituted  for  the  point. 


120  SILICA   AND   THE  SILICATES 

Until  the  fourteenth  century,  nearly  all  the  gems  were 
smoothly  rounded,  as  opals  and  carbuncles  are  now,  or  in 
the  form  of  beads  drilled  from  opposite  sides.  The  cutting 
into  plane  facets  is  quite  modern,  and  is  now  almost  always 
done  with  transparent  stones. 

Gem  cutting  and  engraving  in  modern  times  have  been 
done  with  the  help  of  a  rapidly  rotating  lathe,  carrying  a 
soft  iron  point  or  small  disc  of  soft  iron,  with  diamond  dust 
and  oil,  the  discs  ranging  from  the  size  of  a  pin's  head  to 
£  in.  in  diameter.  The  S.  S.  White  dental  engine,  first 
recommended  for  the  purpose  by  G.  F.  Kunz,  is  even  better 
than  the  lathe,  because  of  the  flexibility  and  sensitiveness 
of  the  machine.  Applied  in  this  way,  diamond  dust  and  oil 
will  carve  any  stone  other  than  diamond  fairly  easily. 

Garnets  have  been  cut  from  very  early  times.  In 
more  recent  periods,  the  cutting  of  garnets  has  been  mainly 
done  in  Bohemia,  where  the  red  pyrope  or  Bohemian  garnet 
is  abundant  enough  to  maintain  an  industry  in  which  were 
engaged  about  500  miners,  as  many  cutters,  and  something 
like  3000  dealers. 

At  Jeypore  (where  many  natives  taught  by  Europeans 
are  employed)  and  other  places  in  India,  garnet  cutting 
is  also  carried  on  extensively,  but  in  this  case  mostly 
almandine  (another  variety  of  red  garnet),  which  occurs 
in  larger  pieces  than  pyrope.  The  same  variety  occurs 
in  large  masses  in  what  was  German  Bast  Africa. 

Ornamental  flat  plates  and  dishes,  etc.,  have  been  carved 
out  of  single  garnets.  Ornamental  dishes  and  other  articles 
have  also  been  carved  out  of  rock  crystal,  agate,  jasper,  etc. 
As  previously  noted,  the  agate  industry  has  been  chiefly 
centred  along  the  Nahe  river  in  Germany. 

For  the  grinding,  large  horizontal  wheels  are  used, 
about  6  ft.  in  diameter  and  ij  ft.  thick,  which  are  worked 
by  water-wheels.  Some  of  these  grindstones  have  faces  with 
grooves  of  different  sizes,  so  that  round  objects  or  convex 
surfaces  can  be  ground  easily  and  quickly.  Thus  an  agate 
ball  is  made  from  a  piece  of  about  the  right  size  (broken 
from  a  larger  piece  if  necessary)  and  held  in  a  semicircular 


SILICATES  121 

groove  until  half  of  it  is  ground  into  shape  ;  it  is  then 
turned  over  for  the  other  half  to  be  similarly  ground.  The 
polishing  is  done  on  wheels  of  wood  with  tripoli  found 
locally  ;  a  wheel  edge  or  drill  is  then  used  to  produce  any 
carving  or  ornamentation  which  may  be  desired.  The 
same  method  is  adopted  for  making  spheres  of  quartz. 

Similar  work  has  been  done  on  a  large  scale  in  the 
United  States  (at  Sioux  Falls,  S.  Dakota),  including  the 
production  of  polished  table  tops  and  columns  of  agatized 
wood.  Fine  lapidary  work  has  also  been  done  in  Russia, 
chiefly  at  Ekaterinburg,  Peterhof,  and  Kolyvan  (Siberia), 
as  well  as  in  France,  China,  Japan,  etc. 

Cutting  Precious  Stones. — In  the  case  of  the  more 
valuable  stones  it  is  often  necessary  to  remove  flaws  from 
the  natural  stones  in  such  a  way  as  to  reduce  the  size  of  the 
gem  as  little  as  possible.  This  is  effected  in  diamonds  by 
carefully  cleaving  or  dividing  them  along  certain  natural 
planes  of  weakness,  which  run  parallel  to  the  octahedral 
faces  of  the  crystals  ;  an  alternative  modern  process  is 
sawing,  which  has  been  largely  adopted.  In  sawing,  the 
stone  is  set  in  a  small  metal  vessel  full  of  melted  aluminium, 
with  only  the  part  to  be  cut  exposed  ;  it  is  then  pressed 
firmly  against  the  edge  of  a  rapidly  rotating  metallic  disc 
or  thin  wheel  (4  or  5  ins.  across)  charged  with  diamond  dust 
and  oil.  Then  either  by  hand  or  by  machinery  two  diamonds 
cemented  at  the  ends  of  supports  serving  as  handles  are 
rubbed  together  until  the  required  general  shape  is  obtained. 
This  preparation  is  not  necessary  for  the  softer  stones  before 
proceeding  to  cut  the  facets.  The  prepared  diamonds  are 
embedded  in  a  fusible  alloy  at  the  end  of  a  handle,  leaving 
exposed  only  the  part  to  be  ground  off,  and  two  mounted 
diamonds  are  rubbed  together  until  a  facet  is  produced. 
The  alloy  is  then  softened,  and  the  stone  set  again  for  grinding 
another  facet,  and  so  on  until  all  are  made.  The  final 
polishing  is  effected  on  horizontal  iron  wheels  rotating  very 
rapidly.  Diamond  dust  (saved  from  previous  operations) 
and  oil  are  applied,  and  the  facets  are  polished  carefully 
one  by  one,  either  by  hand  or  with  the  aid  of  mechanism. 


122  SILICA   AND  THE  SILICATES 

The  cutting  of  other  precious  stones  is  accomplished  by 
similar  wheels,  etc.,  to  those  used  for  cutting  diamonds,  but 
the  wheels  are  made  of  softer  materials,  and  it  is  rarely 
necessary  to  use  diamond  dust  to  cover  them.  The  discs 
or  wheels  used  by  ordinary  lapidaries  are  generally  of  lead, 
tin,  zinc,  copper  or  hard  wood,  and  the  powders  used  include 
emery  (mainly  composed  of  alumina),  tripoli  or  rottenstone 
(silica),  tin  putty  (tin  dioxide),  and  rouge  (anhydrous  ferric 
oxide),  with  carborundum,  alundum,  or  other  modern 
artificial  abrasive  in  cases  where  something  harder  than 
emery  is  desirable.  Most  of  the  colourless  precious  stones 
are  cut  with  a  lead  wheel  and  well  moistened  rottenstone, 
and  this  also  gives  the  first  polish  to  the  silicious  precious 
stones,  like  jasper,  agate,  etc. 

Jewelled  Bearings  for  Watches  were  used  first  about 
the  beginning  of  the  eighteenth  century.  Diamonds  and 
sapphires  are  preferred,  and  are  pierced  either  by  diamond 
drills  or  by  drills  covered  with  diamond  dust.  Sapphires 
and  rubies  can  be  made  artificially  for  the  purpose  at 
moderate  rates. 


LITERATURE. 

Journal  of  the  Society  of  Chemical  Industry. 
Transactions  of  the  Ceramic  Society. 
"  Glossary  of  Mineralogy,"  by  H.  W.  Bristow. 

For  details  of  Carborundum  and  Carbide  Manufacture  see  Rideal's 
"  Electrometallurgy,"  this  series,  pp.  164-181. 

Also  the  works  mentioned  at  the  end  of  Section  T.  (p.  45). 


SECTION  III.— LIME,  MORTAR,  AND  CEMENT 

Materials  used  in  preparation  of  mortars,  etc.,  fall  into 
two  chief  classes  : — 

A.  Those  which  set  only  in  the  air. 

B.  Those  which  set  either  in  the  air  or  under  water  ; 
from  their  ability   to  set  under  water  these   are  termed 
hydraulic  cements,  etc. 

Under  A  are  included  : — 

1.  Gypsum,  which  has  to  be  powdered  by  mechanical 
means,  and  sets  by  taking  up  into  combination  again  water 
which  previously  has  been  removed. 

2.  Lime,   which  on  addition  of  water  crumbles  to  a 
powder  consisting  of  calcium  hydroxide  (slaked  lime),  and 
when  mixed  with  sand  hardens  to  form  ordinary  mortar. 

Under  B  are  included  : — 

3.  Hydraulic  Limes,  produced  by  the  action  of  heat  on 
limestones  containing  clay  and  silica ;   they  fall  to  powder 
when  slaked  with  water. 

4.  Roman  Cement,  produced  by  heating  very  calcareous 
marls  below  the  sintering  temperature,  and  grinding  ;   this 
does  not  slake  with  water. 

5.  Portland   Cement,    produced    by    heating    to    the 
sintering  point  either  suitable  calcareous  marls  or  artificial 
mixtures  of  lime  and  clay.     It  contains  about  2  (1*8  to  2 -2) 
parts  of  lime  to  i  part  of  silica,  alumina,  and  ferric  oxide 
taken  together,  and  its  specific  gravity  exceeds  3. 

6.  Hydraulic  Mixtures,  natural  or  artificial  materials 
which  harden  in  conjunction  with  quicklime,  but   not   by 
themselves.     They  include  pozzuolana,  trass,  blast-furnace 
slags,  etc. 

123 


124  SILICA   AND   THE  SILICATES 

7.  Pozzuolana  Cements,  produced  by  very  intimately 
mixing  finely  divided  slaked  lime  with  trass,  etc. 

8.  Mixed  Cements,  prepared  cements  to  which  other 
substances  have  been  added. 

Lime  or  calcium  oxide  occurs  abundantly  in  nature, 
chiefly  in  combination  with  carbon  dioxide  (carbonic  acid) 
forming  calcium  carbonate,  as  limestone,  marble,  chalk, 
coral,  calcite  and  Iceland  spar,  aragonite,  and  in  the  shells 
of  many  animals  living  in  the  sea,  fresh-water,  and  land. 
As  calcium  phosphate  it  occurs  in  bones  and  also  hi  apatite 
and  other  minerals,  and  as  calcium  sulphate  in  gypsum,  etc. 

The  technical  applications  of  calcium  carbonate  are 
numerous.  Marble  is  used  in  building,  Iceland  spar  is 
used  for  certain  optical  instruments,  chalk  is  used  for 
certain  colours  and  drawing  materials  and  for  making 
marks  on  blackboards  and  slates.  Other  uses  of  calcium 
carbonate  are  in  the  manufacture  of  artificial  mineral 
waters,  etc.,  and  for  certain  chemical  manufactures.  It  is 
also  largely  used  as  a  flux  in  metallurgical  operations, 
especially  in  iron  smelting. 

Calcium  carbonate  is  insoluble  in  pure  water,  but  when 
carbon  dioxide  (carbonic  acid)  is  dissolved  in  the  water,  the 
soluble  calcium  bicarbonate  is  formed.  When  the  soluble 
bicarbonate  loses  half  its  carbonic  acid  by  evaporation,  the 
insoluble  carbonate  is  deposited.  This  action  in  nature 
gives  rise  to  the  formation  of  stalactites  and  stalagmites, 
and  in  like  manner  calcareous  sinter  is  deposited  in  lime- 
stone caverns,  and  on  objects  about  wells  whose  water  has 
passed  through  limestone.  At  a  white  heat,  calcium 
carbonate  is  decomposed,  carbon  dioxide  being  given  off, 
whilst  quicklime  (caustic  lime,  or  calcium  oxide)  remains. 

Lime-Burning. — The  object  of  this  process  is  to  decom- 
pose the  calcium  carbonate  of  the  limestone,  and  to  drive 
off  the  liberated  carbon  dioxide  and  also  the  water  asso- 
ciated with  the  stone.  In  ordinary  practice  a  temperature 
of  about  1000°  C.  is  maintained — it  should  in  any  case  not 
be  outside  the  range  812°  to  1100°  C. — until  all  the  escaping 
gas  has  been  removed,  for  the  action  is  reversible,  the  lime 


LIME,  MORTAR,  AND  CEMENT  125 

being  capable  of  recombining  with  the  carbon  dioxide  to 
form  carbonate  again.  The  natural  draught  of  the  kiln, 
assisted  by  the  water  vapour  formed,  serves  to  remove  the 
carbon  dioxide,  which  is  generally  wasted  in  British  works, 
but  elsewhere  (as  in  the  United  States  and  Germany)  it  is 
collected  for  use.  Water  is  sometimes  added  purposely. 
The  water  vapour  not  only  helps  to  drive  out  the  carbon 
dioxide  but  by  diluting  it  promotes  the  decomposition  of 
more  limestone. 

The  time  required  for  complete  removal  of  carbon 
dioxide  depends  on  the  size,  the  specific  gravity,  and  the 
moisture  of  the  pieces  of  stone.  It  is  better  to  break  the 
limestone  up  rather  small,  but  if  this  would  be  too  expensive, 
the  larger  pieces  are  placed  where  they  will  be  subjected  to 
most  heat.  The  large  size  of  the  stones  usually  necessitates 
a  much  higher  temperature  in  a  kiln  than  is  actually  required 
for  the  decomposition. 

In  the  case  of  limestones  containing  clay  the  latter 
gives  off  additional  water  vapour,  and  removal  of  the 
carbon  dioxide  is  also  promoted  by  the  tendency  of  the 
lime  to  combine  with  silica  and  alumina,  as  well  as  with 
any  ferric  oxide  present. 

There  are  two  classes  of  lime  kilns,  intermittent  kilns, 
in  which  the  calcined  material  is  withdrawn  after  cooling, 
and  a  fresh  charge  put  in,  and  continuous  kilns,  in  which 
the  raw  material  is  fed  in  at  the  top  as  required,  and  the 
lime  is  removed  at  the  bottom.  Continuous  kilns  include 
stationary  or  vertical  kilns,  and  rotary  kilns. 

Vertical  (or  stationary)  kilns  may  have  a  mixed  feed  (in 
what  are  called  running  kilns]  or  separate  feed. 

Among  intermittent  kilns  the  simplest  is  a  flare  kiln, 
built  of  the  limestone  itself,  forming  an  arch  or  arches  at 
the  bottom,  under  which  the  fuel  (mostly  wood  or  peat)  is 
burned.  Such  kilns  are  often  built  of  brick  or  stone-work 
with  an  inner  lining  of  firebricks. 

Vertical  continuous  kilns  are  taller  than  flare  kilns,  and 
when  the  limestone  and  fuel  are  introduced  in  alternate 
layers  the  kilns  are  called  running  kilns.  These,  when 


126  SILICA   AND   THE  SILICATES 

properly  managed,  are  more  economical  than  the  preceding, 
but  the  resulting  lime  is  contaminated  with  the  ashes  of  the 
fuel.  Special  forms  of  running  kilns  include  the  Aalborg 
kiln,  Dietzsch  kiln,  etc.,  which  have  distinct  pre-heating, 
burning,  and  cooling  zones. 

Vertical  continuous  kilns  with  separate  feed  are  much 
used  in  America  and  abroad.  The  fuel  is  burned  in  fire- 
places separate  from  the  kiln  proper.  The  Riidersdorf  kiln 
has  a  high  central  shaft.  Rumford's  kiln  is  a  modified 
form.  Another  type  is  Schmatolla's  kiln,  in  which  generators 
(or  producers)  are  used  instead  of  fireplaces.  In  the  Hoff- 
mann ring  kiln,  which  as  a  whole  is  continuous,  each  of  the 
chambers  is  practically  an  intermittent  kiln.  It  is  very 
economical  as  regards  fuel,  but  is  costly  in  labour.  It  is 
chiefly  used  on  the  Continent. 

Rotary  Kilns  are  sometimes  used  for  the  burning  of 
lime,  the  temperature  being  much  lower  than  for  cement. 

Properties  of  Lime. — When  a  limestone  is  nearly  pure 
calcium  carbonate,  the  lime  obtained  from  it  is  called  a 
"  fat "  or  "  rich "  lime.  When  the  limestone  contains 
notable  proportions  of  magnesium  carbonate,  the  lime 
produced  from  it  forms  with  water  a  thin  "  milk  of  lime," 
and  is  said  to  be  "  lean  "or  "  poor."  I^ime  is  distinctly 
poor  when  it  has  10  per  cent,  of  magnesia,  and  25  to  30  per 
cent,  renders  it  almost  useless.  Fat  lime  when  slaked  with 
water  expands,  gives  off  much  hot  vapour,  and  quickly  falls 
to  powder.  I/ean  lime  falls  to  powder  within  a  few  minutes, 
but  there  is  little  heat,  and  scarcely  any  expansion.  Some 
pieces  of  limestone  are  only  burned  on  the  surface,  and 
contain  unaltered  limestone  within,  whilst  other  pieces 
are  overburned.  Overburning  may  cause  superficial  sinter- 
ing of  the  little  silica  and  alumina  present,  a  coating  of 
silicate  thus  preventing  the  lime  within  from  combining 
with  water  to  form  a  cream,  or  a  half-burned  lime  may  be 
formed  by  a  sudden  strong  heating,  in  which  only  part  of 
the  carbonate  is  decomposed. 

Pure  quicklime  is  white  and  amorphous,  but  it  can  be 
obtained  in  a  crystalline  form ;  the  specific  gravity  of  the 


LIME,  MORTAR,  AND  CEMENT  127 

latter  is  3-25,  and  that  of  the  amorphous  lime  3-15.  lyime 
melts  only  at  a  very  high  temperature,  but  it  is  strongly 
basic,  and  when  highly  heated  it  combines  with  silica  or 
alumina.  Water  added  to  quicklime — i  volume  to  about 
3  volumes,  or  about  32  parts  to  100  parts  (18  to  56)  by 
weight— causes  it  to  slake  very  violently,  with  great  evolution 
of  heat.  Any  excess  of  water  is  driven  off  as  vapour.  The 
slaked  lime  is  a  soft  white  powder,  which  occupies  more  than 
three  times  the  volume  of  the  original  quicklime,  and  consists 
of  calcium  hydroxide,  CaH2O2.  If  less  water  be  added,  a 
sandy  powder  is  obtained,  which  has  little  technical  value. 
Hence  lime  should  not  be  left  in  damp  places.  For  building 
purposes,  after  slaking  the  lime  with  about  one-third  its 
weight  of  water,  an  equal  quantity  of  water  is  added  to 
form  a  thin  cream.  Further  addition  of  water  to  this 
cream  gives  milk  of  lime  or  lime-water,  a  saturated  solution 
of  the  hydroxide.  The  solubility  varies  according  to  the 
mode  of  preparing  the  lime  or  calcium  hydroxide. 

Slaked  lime  can  be  heated  to  250°  or  300°  C.  without 
losing  any  water,  but  at  530°  to  540°  C.  water  is  driven  off 
and  quicklime  left.  L,ime- water  is  a  strongly  alkaline 
solution,  which  turns  red  litmus  blue,  and  readily  absorbs 
carbon  dioxide  to  form  calcium  carbonate.  I4me  exposed 
to  the  air  gradually  becomes  slaked  by  absorbing  moisture, 
part  of  it  being  converted  into  carbonate.  Air-slaked  lime 
has  a  composition  nearly  represented  by  CaCO3.CaH2O2. 

Calcium  hydroxide  mixed  with  a  little  water  forms  a 
paste  which,  when  left  exposed  to  the  air,  dries  and  shrinks, 
leaving  a  not  very  hard  porous  mass.  The  paste  slowly 
takes  up  carbon  dioxide  from  the  air,  gradually  forming  a 
harder  crust  of  calcium  carbonate,  but  this  action  does  not 
extend  to  the  interior.  When  protected  from  the  air,  the 
paste  or  dry  powder  equally  remains  unchanged,  even  after 
many  years.  Setting  of  pure  slaked  lime  depends  on  this 
property  of  drying  to  a  cake  of  moderate  hardness,  as  in 
the  case  of  other  finely  divided  amorphous  materials. 
Hardening  of  the  lime  results  from  later  absorption  of 
atmospheric  carbon  dioxide,  though  the  coating  of  calcium 


128  SILICA   AND   THE  SILICATES 

carbonate  first  formed  generally  prevents  further  action 
within  it. 

Ivimes  for  building  purposes  are  made  from  limestones 
containing  as  impurities  clayey  and  silicious  materials, 
iron  oxides,  alkalies,  etc.,  the  amounts  of  which  affect  the 
behaviour  of  the  lime.  When  such  impure  limes  are  calcined, 
the  lime  enters  into  combination  with  the  constituents  of 
the  impurities  present ;  thus  with  clayey  limestones  the 
lime  combines  with  silica  and  alumina  of  the  clay  to  form 
calcium  silicates  and  calcium  aluminates,  and  it  also 
combines  with  any  iron  oxides  present  to  form  calcium 
ferrite. 

For  making  products  known  as  "  hydrated  lime,"  "  new 
process  lime,"  etc.,  the  quicklime  is  ground,  the  powder 
thoroughly  slaked  with  water,  and  the  product  either  sieved 
or  passed  through  air  separators,  to  give  a  uniformly  fine 
powder.  The  slaking  is  sometimes  done  by  simply  sprinkling 
with  water,  but  to  obtain  more  thorough  saturation  special 
appliances — such  as  the  Kritzer  hydrator — are  often  used. 

Hydraulic  Lime. — The  process  of  slaking  lime  is 
much  modified  by  the  presence  of  other  substances,  and 
may  even  take  place  without  noticeable  evolution  of  heat 
and  with  insignificant  change  of  volume.  The  hydrated 
calcium  silicates  and  aluminates  have  the  power  of  setting 
and  hardening  under  water,  and  a  lime  containing  enough 
of  these  compounds  to  thus  set  and  harden  is  termed  a 
hydraulic  lime,  or  water  lime  (the  old  name).  Limestone 
with  more  than  10  per  cent,  silica  usually  gives  on  calcination 
a  hydraulic  lime.  If  only  moderately  hydraulic  the  lime  when 
slaked  with  water  may  show  no  action  until  5  to  15  minutes, 
after  which  steam  is  given  off,  but  less  abundantly  and  with 
less  manifestation  of  heat  than  with  ordinary  lime  (either 
fat  or  lean)  ;  such  a  hydraulic  lime  would  set  under  water 
in  15  to  20  days,  but  would  harden  further  for  some  months  ; 
it  may  require  2  days  or  more  to  slake,  whilst  more  hydraulic 
limes  might  need  7  to  14  days.  Ordinary  or  normal  hydraulic 
limes  after  treatment  with  water  remain  inactive  for  an 
hour  or  more,  and  subsequently  do  not  give  off  much  steam 


LIME,  MORTAR.   AND  CEMENT  129 

or  heat,  but  fall  to  powder ;  these  limes  set  in  water  after 
6  to  8  days,  and  harden  further  for  a  few  months.  Very 
hydraulic  limes  remain  inactive  for  indefinite  periods,  and 
evolution  of  heat  is  hardly  noticeable,  while  the  material 
does  not  readily  fall  to  powder  ;  these  limes  set  under  water 
within  3  or  4  days,  and  are  quite  hard  after  a  month,  but 
harden  further. 

Some  French  limestones  contain  finely  divided  silica, 
which  in  burning  combines  with  the  lime,  as  in  the  Chaux  de 
Theil  (made  from  a  limestone  found  at  Ard£che),  which 
contains  65  to  75  per  cent,  lime,  20  to  22  silica,  about  2  per 
cent,  alumina,  with  a  little  magnesia,  ferric  oxide,  etc.,  and 
has  been  much  used,  especially  for  marine  construction. 

In  this  country  hydraulic  limes  are  generally  sold  in 
lumps  (as  obtained  from  the  kiln).  French  hydraulic  limes 
are  slaked  and  sieved  at  the  lime  works.  The  hard  lumps 
which  will  not  pass  through  the  sieves  are  partly  useless 
unchanged  limestone  and  partly  calcium  silicates.  The 
lumps  are  collected  and  finely  ground  to  form  what  is  called 
in  England  and  America  grappier  cement.  The  well-known 
La  Farge  cement,  much  used  in  the  United  States,  belongs  to 
this  class.  In  France,  part  of  the  ground  lumps  is  mixed 
with  the  lime  to  increase  its  hydraulic  properties. 

Calcium  Sulphate  occurs  naturally  in  several  forms. 
In  the  anhydrous  condition  it  is  the  mineral  anhydrite, 
which  is  often  found  in  crystals  and  in  semi-crystalline 
masses  in  limestones  or  associated  with  common  salt  deposits. 
The  sulphate  is  much  more  abundant  as  the  hydrated  mineral 
gypsum,  CaSO4.2H2O,  which  occurs  in  more  or  less  trans- 
parent crystals  called  selenite,  which  crystals  easily  split 
into  thin  laminae,  have  hardness  1-5  to  2,  and  specific 
gravity  2*28  to  2-33.  Other  special  forms  are  the  fibrous 
satin-spar  and  the  granular  crystalline  alabaster.  These 
two  varieties  are  worked  into  ornaments  and  artistic 
products.  Massive  gypsum  or  plaster  stone  is  a  stratified 
variety  mostly  used  for  making  plaster.  Earthy  gypsum 
consists  of  small  grains  of  gypsum,  with  much  clay  and  sand. 
Gypsum  is  sparingly  soluble  in  water,  requiring  over 
c.  9 


130  SILICA   AND  THE  SILICATES 

400  parts  of  water  for  complete  solution  at  ordinary  temper- 
atures. When  heated  to  100°  to  120°  C.,  gypsum  loses  three- 
fourths  of  its  water  fairly  rapidly,  but  the  remainder  is  only 
driven  off  at  or  above  204°  C.  (400°  F.),  forming  "  soluble 
anhydrite."  When  the  heating  is  continued  further,  the 
dead  burned  or  over-burned  gypsum  no  longer  hardens  by 
combining  with  water,  which  it  takes  up  very  slowly.  The 
anhydrous  sulphate  melts  without  being  decomposed  when 
heated  to  redness,  and  resembles  anhydrite  on  cooling. 
When  gypsum  is  dried  at  about  100°  C.,  the  hemihydrate, 
2CaSO4.H2O,  or  CaSO4.0'5H2O,  is  formed,  the  specific 
gravity  of  which  is  2*7  ;  this  product  constitutes  plaster 
of  Paris,  which  when  mixed  with  water  combines  with 
it  to  form  the  dihydrated  sulphate  again,  solidifying  to  a 
hard  mass,  and  gives  off  heat,  at  the  same  time  expanding 
and  filling  a  mould  in  which  it  is  cast ;  it  is  because  of  this 
that  plaster  of  Paris  is  so  largely  used  for  casting. 

The  water  of  crystallization  in  gypsum  can  be  removed 
by  adding  salts,  as  dilute  solution  of  potassium  sulphate  or 
carbonate  ;  the  hardness  which  results  in  this  case  takes 
place  more  rapidly  than  with  burned  gypsum  and  water. 
When  gypsum  has  been  hardened  in  this  way,  if  it  be 
powdered  and  treated  again  with  the  same  solution  (potas- 
sium sulphate  or  carbonate) ,  it  hardens  again.  This  property 
is  employed  technically  in  re-hardening  old  gypsum,  and 
in  hardening  incompletely -burned  gypsum,  by  using  solution 
of  potassium  carbonate  instead  of  water. 

The  good  qualities  of  plaster  depend  chiefly  on  the  raw 
material  used  and  on  the  method  of  burning  it.  The  best 
commercial  results  are  obtained  from  the  denser  (and  there- 
fore heavier)  varieties  of  gypsum.  The  lowest  temperature 
at  which  gypsum  can  be  burned  to  plaster  is  80°  C.,  but 
from  a  technical  standpoint  the  best  results  are  attained 
at  110°  to  120°  C.  As  a  rule,  the  smaller  the  pieces  of 
gypsum,  the  more  uniform  the  product. 

Burning  of  Gypsum. — Gypsum  is  burned  in  ovens  or 
kilns  in  such  a  way  as  to  avoid  the  reduction  to  sulphide  by 
the  fuel. 


LIME,  MORTAR,  AND  CEMENT          131 

A  common  form  of  kiln  on  the  continent  consists  of 
strong  walls  crossed  by  a  low  arch  with  ventilating  openings. 
The  gypsum  only  is  placed  in  the  upper  space,  above  small 
arched  fire-chambers  in  which  brushwood  is  burned,  or 
sometimes  coke  or  coal.  The  arrangement  is  very  simple, 
but  wasteful  of  raw  material  and  fuel.  Scanegatty's  kiln 
is  essentially  similar,  but  the  walls  and  roof  arch  are  much 
thicker,  and  the  series  of  small  fire  arches  is  replaced  by  a 
single  low  arch  about  a  foot  above  the  floor ;  the  flames 
from  a  furnace  connected  with  the  lower  chamber  strike 
against  the  under  side  of  the  arch,  and  the  hot  air  and  gases 
pass  through  openings  into  the  upper  chamber.  The  water 
vapour  and  waste  gases  pass  out  through  a  central  opening 
at  the  top  of  the  kiln.  Dumesnirs  kiln,  which  is  economical 
in  working  (though  expensive  to  build),  and  has  been  much 
used,  differs  from  Scanegatty's  chiefly  in  the  arrangement 
of  the  lower  fire-chamber,  which  has  twelve  openings  to  the 
upper  chamber,  and  passages  continued  along  this  to  the 
walls  by  the  arrangement  of  large  blocks  of  gypsum ;  the 
lower  layers  of  gypsum  are  disposed  so  as  to  offer  every 
facility  for  the  draught  to  circulate.  The  firing  is  conducted 
gently  for  four  hours,  then  more  vigorously  for  eight  hours, 
after  which  all  the  openings  are  closed,  and  5  to  6  cubic 
metres  of  coarse  gypsum  powder  spread  evenly  over  the  top 
of  the  gypsum  in  the  kiln,  thus  burning  more  material 
and  saving  fuel.  After  twelve  hours'  cooling  the  gypsum  is 
all  removed.  If  not  in  fairly  uniform  powder,  it  must  be 
ground,  usually  in  a  stamp  or  roller  mill.  The  powdered 
gypsum  is  sifted,  and  stored  in  a  dry  place.  Sometimes  the 
operations  of  grinding  and  sifting  are  combined  in  one 
apparatus. 

Improvements  have  been  introduced  by  making  the 
process  continuous,  and  securing  more  equal  distribution 
of  heat.  Uniform  dehydration  is  also  facilitated  by  grinding 
the  gypsum  before  calcining  it.  Petry  and  Hecking's  kiln 
is  a  rotary  kiln.  Perm's  barrel  kiln  is  a  revolving  cylinder 
on  hollow  trunnions,  through  one  of  which  the  heated  gases 
from  the  furnace  enter  while  the  other  serves  for  exit.  It 


132  SILICA   AND  THE  SILICATES 

is  charged  through  a  door  in  the  side  of  the  cylinder,  and  the 
burned  product  is  discharged  at  the  same  place. 

Some  kilns  are  heated  by  means  of  steam,  a  jet  of  steam 
at  about  200°  C.  being  blown  alternately  into  each  of  two 
chambers  containing  raw  gypsum.  Waste  gases  from  coke 
ovens  are  also  used  to  dehydrate  gypsum. 

The  best  quality  of  plaster  is  sometimes  made  on  a 
small  scale  by  "  boiling,"  the  finely  powdered  gypsum 
forming  a  layer  2  or  3  ins.  deep  upon  a  metal  plate  or  shallow 
dish,  which  is  placed  over  a  fire  in  such  a  way  as  not  to  be 
over-heated ;  the  material  is  stirred  at  intervals  until  no 
more  steam  escapes,  and  the  plaster  is  then  ready  for  use. 
In  the  United  States  plaster  is  often  prepared  in  a  similar 
way  by  "  boiling "  ground  gypsum  in  cylindrical  iron 
kettles,  with  constant  stirring  by  machinery.  The  method 
is  slow  and  expensive,  though  superior  to  the  older  European 
processes,  so  calcination  of  roughly  crushed  gypsum  in 
rotary  kilns  is  gradually  superseding  it.  The  Cummer 
rotary  calciner  is  largely  employed  in  the  United  States. 
The  Mannheim  calciner  is  a  rotating  cylinder  provided 
with  a  forewarmer,  and  the  heat  is  so  well  used  up  that  the 
escaping  gases  have  a  temperature  of  only  about  80°  C. 

Gypsum  when  heated  to  higher  temperatures,  as  above 
130°  C.,  becomes  converted  into  anhydrous  calcium  sulphate, 
which  appears  to  exist  in  two  or  more  modifications,  the 
behaviour  of  which  with  water  is  different.  Eventually 
all  varieties  become  hydrated,  but  they  do  not  all  set  as 
plaster. 

Keene's  Cement,  and  Flooring  Plaster  or  Estrich- 
gips,  consist  essentially  of  anhydrous  sulphate,  and  their 
setting  is  affected  by  the  burning  temperature  and  by  the 
presence  of  minute  proportions  of  other  substances  in  the 
calcium  sulphate,  but  the  nature  of  the  action  is  not  well 
understood.  However  they  may  be  prepared,  the  final 
product  of  setting  is  the  dihydrated  sulphate,  CaSO4.2H2O. 

Keene's  Cement  is  usually  made  in  Britain  by  burning 
gypsum  to  plaster,  dipping  the  burned  lumps  in  a  solution 
of  alum  or  aluminium  sulphate,  then  drying,  and  burning 


LIME,  MORTAR,  AND  CEMENT  133 

again  at  about  500°  C.,  avoiding  contact  with  the  fuel,  and 
finally  grinding  again.  Typical  Keene's  cement  is  nearly 
pure  CaSO4,  with  a  small  percentage  of  carbonate,  the 
added  alum,  etc.,  being  present  only  in  traces.  It  takes 
several  hours  to  set.  Sometimes  the  first  calcination  takes 
place  at  a  high  temperature,  and  sometimes  alum  is  replaced 
by  some  other  salt. 

Flooring  Plaster  fEstrichgips)  is  made  by  burning 
coarsely  crushed  gypsum  at  about  400°  to  500°  C.  for  not 
more  than  four  hours.  I^onger  heating  causes  the  plaster 
to  lose  its  power  of  setting.  After  calcining,  the  flooring 
plaster  is  ground  very  finely.  Such  plasters  set  very  slowly, 
but  eventually  become  very  hard,  and  are  much  used  in 
Germany  for  floors,  etc.  Prolonged  heating  at  high  temper- 
ature, or  more  intense  heating  for  a  shorter  time,  causes 
dehydrated  gypsum  to  gradually  lose  its  power  of  combining 
with  water. 

Plaster  of  Paris  sets  in  a  few  minutes  when  pure.  In 
order  to  cause  slower  setting,  retarders  such  as  glue,  blood, 
vegetable  juices,  etc.,  are  often  added  to  plaster,  and  probably 
act  by  interfering  with  the  growth  of  crystals.  From 
2  to  15  Ib.  of  retarder  per  ton  of  plaster  is  usually  added, 
and  well  mixed.  Occasionally  accelerators  are  added  to 
plaster,  sodium  chloride  (common  salt)  being  one  of  the 
best. 

The  chief  uses  of  plaster  prepared  from  calcium  sulphate 
are  for  plastering  interiors  of  buildings,  but  owing  to  its 
solubility  in  water  it  cannot  be  similarly  used  out  of 
doors. 

It  is  also  largely  used  for  making  moulds  and  casts 
which  do  not  require  a  high  temperature,  and  also  for 
preparing  surgical  supports  for  broken  limbs,  for  adding  to 
Portland  cement  to  make  it  set  more  slowly,  for  cementing 
small  objects,  and  for  spreading  over  soil  for  agricultural 
purposes.  Clear  gypsum  is  ground  and  polished  for  cheap 
jewellery.  Gypsum  is  also  much  used  in  paper-making. 

For  making  casts,  i  part  plaster  is  made  into  a  thin 
paste  with  2\  parts  water,  which  should  harden  in  a  minute 


134  SILICA   AND  THE  SILICATES 

or  two,  or  even  more  quickly  with  moderate  heating.  The 
object  is  previously  well  oiled. 

Gypsum  dissolves  easily  in  excess  of  sodium  thiosulphate, 
a  soluble  double  thiosulphate  of  calcium  and  sodium  being 
formed. 

A  mixture  of  equal  weights  of  anhydrous  calcium  sulphate 
and  potassium  sulphate,  when  stirred  up  with  less  than  its 
weight  of  water,  solidifies  suddenly  ;  with  4  to  5  parts  of 
water,  the  solidification  is  somewhat  slower,  but  it  gives 
casts  with  polished  surfaces. 

Hardening  of  Plaster. — Plaster  may  be  hardened  by 
adding  to  it  lime-water,  or  solution  of  gum  arabic  or  glue, 
or  a  decoction  of  mallow  root.  A  very  good  method,  which 
is  much  used,  is  to  mix  the  plaster  with  alum  solution 
(containing  20  oz.  alum  in  6  Ib.  water)  ;  the  mixture  hardens 
completely  in  15  to  30  minutes,  and  is  known  as  marble 
cement.  Alternatively,  the  cast  piece  may  be  left  for  a 
few  weeks  in  an  alum  bath,  and  then  dried  slowly,  or  the 
plaster  itself  may  be  soaked  in  the  solution,  dried  again, 
and  calcined. 

Addition  of  a  little  lime  (as  by  mixing  the  plaster  with 
lime-water)  increases  the  hardness,  and  gives  an  appearance 
resembling  marble ;  such  mixtures  are  much  used  for 
architectural  purposes,  under  the  name  of  stucco.  When 
dry,  the  surface  is  rubbed  down  with  pumice-stone,  usually 
coloured  to  represent  marble,  and  polished  with  Tripoli 
and  olive  oil. 

Stucco  may  also  contain  other  ingredients.  It  is  largely 
used  for  decorative  purposes  in  architecture,  chiefly  indoors. 

Another  method  of  hardening  plaster  is  to  add  a  little 
slaked  lime,  and  to  dip  the  cast  piece  in  a  strong  solution  of 
magnesium  sulphate  (Epsom  Salt). 

Parian  Cement  is  plaster  mixed  with  borax  solution 
(i  part  borax  in  9  parts  water,  or  better  with  addition  of 
i  part  cream  of  tartar).  It  may  be  prepared  like  Keene's 
cement,  using  borax  instead  of  alum. 

Solution  of  water-glass  does  not  act  well  in  hardening 
plaster.  An  artificial  stone  is  obtained  by  Fissot  by  burning 


LIME,   MORTAR,  AND  CEMENT  135 

gypsum,  immersing  it  in  water  for  half  a  minute,  then 
exposing  to  the  air,  and  again  immersing  for  two  to  three 
minutes,  after  which  the  block  appears  as  a  hardened  stone. 
This  method  suggests  a  new  crystallization  as  the  cause  of  the 
increased  hardness. 

Hardened  plaster,  when  treated  with  stearic  acid 
(stearine)  or  with  paraffin  (wax),  and  then  polished,  bears 
a  close  resemblance  to  meerschaum,  and  still  closer  on 
imparting  a  faint  reddish-yellow  tint  by  a  solution  of  gamboge 
and  dragon's  blood.  Cheap  artificial  meerschaum  pipes 
are  made  in  this  way. 

Scott's  Cement  or  Selenitic  Cement  was  produced  by 
the  action  of  sulphur  fumes  on  ignited  lime,  but  it  is  more 
simply  obtained  by  grinding  quicklime  (preferably  feebly 
hydraulic)  with  5  per  cent,  of  plaster.  Selenitic  cement 
sets  rapidly  and  soon  hardens.  It  is  better  than  ordinary 
hydraulic  lime,  but  is  not  suitable  for  use  in  work  exposed 
to  weather  or  salt  water.  The  better  appreciation  of  Port- 
land cement  has  prevented  it  from  coming  into  general  use. 

Tripolite  is  merely  impure  gypsum  (containing  sand, 
with  calcium  and  magnesium  carbonates),  burned  with 
one-tenth  of  its  weight  of  coal  or  coke. 

Hard-finish  Plasters  are  plaster  cements  consisting 
(like  flooring  plasters)  essentially  of  anhydrous  calcium 
sulphate,  but  they  are  calcined  at  even  higher  temperatures 
— sometimes  at  red  heat.  Keene's  cement  and  Parian 
cement  are  of  this  class. 

Martin's  Cement  resembles  Keene's  cement,  but  potas- 
sium carbonate  solution  replaces  alum. 

Mack's  Cement  is  made  by  adding  sodium  sulphate  or 
potassium  sulphate  to  dehydrated  gypsum. 

All  these  plasters  set  quickly,  and  are  hard  and  durable, 
but  they  are  expensive. 

MORTARS. 

Mortar  is  made  by  mixing  sand  with  milk  of  lime. 
Common  Mortar   (ordinary  mortar)   sets  and  hardens 
only  in  the  air.     Slaked  lime  when  exposed  to  the  air  absorbs 


136  SILICA   AND  THE  SILICATES 

carbon  dioxide,  and  the  material  shrinks  greatly  and  cracks. 
The  calcium  carbonate  formed  in  this  way,  when  thoroughly 
dry,  becomes  as  hard  as  marble,  and  so  is  well  suited  for 
binding  together  bricks,  stone  blocks,  etc.  To  avoid  the 
great  irregularities  which  would  result  from  the  shrinking, 
this  is  lessened  by  the  addition  of  sand  or  some  other  suitable 
substance.  Common  mortar  is  usually  prepared  by  Ulti- 
mately mixing  slaked  lime,  sand,  and  water,  preference 
being  given  to  sand  consisting  of  angular  grains  rather 
than  smooth  round  grains,  because  mortar  made  with 
round-grained  sand  is  very  brittle.  Sometimes  instead 
of  sands  for  making  mortar  are  used  burned  clay  (only 
occasionally  raw  clay  for  mud  walls,  etc.),  pozzuolana,  or 
certain  artificially  calcined  products,  such  as  cinders, 
furnace  slags,  etc.  The  quality  and  hardness  of  the  mortar 
depend  much  on  the  proportion  of  sand  to  lime.  Fat  limes 
can  be  mixed  with  sand  of  2  to  2^  times  the  volume  of  the 
quicklime  previous  to  slaking,  whereas  lean  limes  and 
hydraulic  lime  should  be  mixed  with  smaller  proportions 
of  sand,  sometimes  not  more  than  an  equal  bulk  (especially 
for  important  work).  Mortar  made  from  fat  (or  pure) 
lime  is  disintegrated  by  water.  In  bricklaying,  the  surface 
of  the  brick  is  moistened,  mortar  is  laid  between  the  bricks, 
and  left  to  dry.  Small  quantities  of  mortar  are  often 
mixed  by  hand,  but  large  quantities  usually  by  machinery. 

Hardening  of  Mortar. — The  hardening  or  setting  of 
mortar  takes  place  very  rapidly  ;  it  becomes  quite  firm 
in  a  day.  The  setting  of  the  mortar  is  due  simply  to  the 
drying  of  the  lime,  no  chemical  action  being  involved,  and 
the  sand  being  merely  mixed  with  lime.  lyater  hardening 
of  the  mortar  arises  from  gradual  combination  of  atmospheric 
carbon  with  lime,  but  the  action  is  only  superficial,  a  skin  of 
calcium  carbonate  being  produced. 

Hydraulic  Mortar. — As  the  name  implies,  this  contains 
hydraulic  lime  instead  of  ordinary  lime.  Hydraulic  mortars 
are  made  from  milk  of  lime  and  sand,  or  from  ordinary 
air-mortar  mixed  with  cement  and  water.  Water  is  ab- 
sorbed during  the  slaking  of  hydraulic  lime,  but  there  is  no 


LIME,  MORTAR,  AND  CEMENT  137 

notable  rise  of  temperature  or  increase  in  volume.  Good 
hydraulic  mortars  may  be  made  by  mixing  lime  with  furnace 
ashes  or  burned  clay.  They  are  used  in  the  same  way  as 
ordinary  mortars,  the  brickwork  or  masonry  being  moistened. 
The  mortar  should  be  laid  on  thickly  between  the  layers  of 
bricks. 

Hydraulic  limes  form  links  between  ordinary  limes  and 
Roman  cement. 

All  limes,  as  shown  by  Vicat,  lose  much  of  their  strength 
if  mixed  with  excess  of  water.  Hence  it  is  better  to  wet  the 
bricks  and  stones,  and  to  use  a  stiff  mortar,  than  to  use 
very  soft  mortar  like  many  bricklayers  and  masons  do. 
The  system  of  grouting  increases  the  difficulties  of  setting 
owing  to  the  extra  water. 

Mortars  made  in  summer  are  always  less  effective  than 
those  made  in  winter,  probably  because  of  drying  too 
quickly.  Watering  of  brickwork  and  masonry  constructed 
in  summer  is  recommended,  as  a  safeguard. 

The  volume  occupied  by  mortar  may  generally  be 
considered  as  equal  to  about  three-quarters  of  the  total 
volume  of  sand  and  lime  used. 

Cement  Mortar  is  a  mixture  of  sand,  Portland  cement, 
and  water,  but  sometimes  natural  cement  or  slag  cement 
is  used  instead  of  Portland  cement.  Cement  mortar  is  far 
stronger  than  ordinary  mortar,  but  to  make  it  spread  more 
readily  some  lime  is  usually  added  or  sometimes  loam. 
These  additions  reduce  the  strength,  though  builders  do 
not  seem  to  realize  the  fact.  The  materials  for  cement 
mortar  should  be  well  mixed  in  the  dry  condition,  either  by 
hand  or  in  a  mixing  machine,  and  the  water  should  then  be 
sprinkled  on  the  mixture  through  a  rose.  The  sand  should 
not  usually  exceed  3  to  i  of  cement,  and  in  no  case  should 
it  be  more  than  4  to  i. 

Cement  mortar  is  used  for  special  purposes,  as  for 
masonry  exposed  to  running  water  or  to  sea  water,  and  also 
in  large  land  works  where  great  strength  is  necessary. 
Another  use  is  to  cover  the  surface  of  masonry  and  concrete 
walls. 


138  SILICA   AND   THE  SILICATES 

ROMAN  CEMENT. 

Roman  cement  belongs  to  the  class  of  natural  cements 
produced  by  the  calcination  of  certain  argillaceous  limestones, 
and  the  fine  grinding  of  the  product.  These  natural  cements 
represent  extreme  cases  of  hydraulic  limes  containing  so 
little  free  lime  that  water  does  not  cause  disintegration  of 
the  clinker ;  they  consist  essentially  of  silicates  and  alumi- 
nates,  possibly  with  alumino-silicates,  as  do  also  all  other 
hydraulic  calcareous  cements. 

Roman  cement  (originally  called  Parker's  cement)  was 
discovered  by  James  Parker  of  L,ondon.  He  took  out  a 
patent  (No.  2121)  in  1796  for  making  a  cement  from  the 
septaria  nodules  of  the  I^ondon  Clay,  by  calcining  the 
broken  nodules  to  sintering  point,  and  grinding  the  resulting 
clinker.  Similar  nodules  from  various  other  localities 
were  afterwards  used.  The  composition  of  the  stones 
varies  much,  but  they  mostly  included  between  60  and  70 
per  cent,  calcium  carbonate,  14  to  20  per  cent,  silica,  5  to 
10  per  cent,  alumina,  with  varying  amounts  of  iron  oxide 
and  water,  and  sometimes  small  quantities  of  magnesia 
and  other  substances.  The  raw  stone  is  fine  grained,  yet 
so  porous  that  it  cleaves  to  the  tongue.  Calcination  changes 
the  colour  from  (usually)  greyish  to  brownish,  and  increases 
the  porosity,  while  about  a  third  of  the  weight  is  lost.  The 
powder  is  generally  reddish  brown,  and  has  a  specific 
gravity  of  2*5  to  3*0. 

Calcination  is  now  carried  out  at  somewhat  lower  temper- 
atures than  originally — about  a  moderate  red  heat — for  it 
has  been  ascertained  that  sintering  is  not  necessary  (as 
Parker  supposed).  This  has  the  effect  of  reducing  the  cost 
of  grinding.  The  clinker  may  be  kept  for  a  long  time 
without  becoming  changed,  and  it  takes  up  water  with 
great  difficulty.  After  crushing  and  fine  grinding,  it  should 
be  used  as  soon  as  possible,  as  moist  air  causes  rapid 
deterioration. 

Roman  cement  generally  contains  between  45  and  60 
per  cent,  of  lime,  about  3  magnesia,  5  to  10  alumina,  5  to 


LIME,   MORTAR,   AND   CEMENT  139 

12  ferric  oxide,  and  20  to  30  silica,  with  i  to  2  per  cent, 
manganese  oxide.  It  sets  very  rapidly,  the  exact  time  of 
setting  depending  on  the  temperature  and  quantity  of 
water  used,  and  also,  when  exposed,  to  the  temperature 
and  hygroscopic  condition  of  the  air.  A  neat  Roman 
cement  of  good  quality,  gauged  with  water  at  15°  C.,  will 
set  in  air  in  about  5  or  10  minutes,  and  under  water  in 
15  to  60  minutes.  Setting  proceeds  more  slowly  in  sea 
water,  and  is  also  retarded  by  addition  of  plaster  of  Paris. 

Mixtures  of  Roman  cement  and  sand  (hydraulic  mortars) 
set  more  slowly  than  the  cement  alone,  but  have  much  less 
strength.  The  strength  and  hardness  of  Portland  cement 
are  far  greater,  and  so  it  is  much  more  extensively  used. 
Owing  to  its  rapid  setting,  Roman  cement  proves  useful 
for  protecting  marine  work  constructed  of  Portland  cement 
or  concrete. 

Natural  cements,  more  or  less  resembling  Roman  cement, 
have  been  made  on  the  Continent,  and  in  the  United  States, 
some  of  them  being  termed  natural  Portland  cements.  They 
are  inferior  in  strength. 

Roman  cement  and  similar  products  differ  from  Portland 
cement  in  having  more  variable  composition,  in  setting 
more  quickly,  but  having  much  lower  strength  and 
hardness. 


PORTLAND  CEMENT. 

Portland  cement,  when  set,  presents  an  appearance 
similar  to  Portland  stone,  hence  the  name.  It  was  first 
made  in  1824,  by  Joseph  Aspdin,  of  I,eeds,  who  mixed 
well  equal  weights  of  clay  and  finely-ground  limestone,  and 
calcined  the  mixture  in  a  lime  kiln.  The  product,  being 
rather  lightly  calcined,  was  very  different  from  modern 
Portland  cement.  In  1826,  General  Pasley  prepared  a 
cement  by  calcining  a  mixture  of  river  mud  (from  the 
Medway)  with  limestone  or  chalk.  The  special  suitability 
of  Medway  mud  is  probably  owing  to  the  presence  of  salts 
derived  from  sea  water,  and  mud  from  about  the  mouths 


140 


SILICA   ANDJ THE 'SILICATES 


of  other  rivers   are   similarly   used   for   making   Portland 
cement. 

In  general,  Portland  cement  is  obtained  by  grinding 
finely  the  clinker  produced  by  strongly  heating  an  intimate 
mixture  of  calcareous  and  argillaceous  materials  ;  the  heat 
should  cause  sintering  (incipient  fusion).  The  choice  of 


FIG.  2. — The  Bradley  three-roll  mill. 

raw  materials  may  bermade  from  limestone,  chalk,  cement 
rock  (an  argillaceous  limestone  softer  than  pure  limestone, 
with  50  to  80  per  cent,  calcium  carbonate,  but  with  not 
more  than  4  per  cent,  magnesium  carbonate  for  Portland 
cement),  chalk  marl,  clay  shale,  slate,  blast-furnace  slag, 
alkali  waste  (material  produced  in  the  manufacture  ol 
caustic  soda,  but  it  should  not  contain  sulphur),  etc.,  but 


LIME,  MORTAR,  AND  CEMENT          141 

the  composition  of  the  product  must  be  kept  within  narrow 
limits ;  the  mixture  should  be  finely  divided,  uniform,  and 
should  contain  about  75  per  cent,  of  calcium  carbonate 
and  25  per  cent,  of  aluminium  silicate  and  free  silica  with 
iron  oxide  (to  include  not  more  than  5  per  cent,  of  magnesia, 
alkalies,  and  sulphur  trioxide).  The  limestone  for  cement 
should  not  contain  more  than  2  per  cent,  magnesia. 

Three  processes  are  in  use  for  mixing  the  raw  materials  : 
the  wet,  semi-wet  or  semi-dry,  and  dry  processes. 

The  wet  process  is  General  Pasley's  original  process, 
and  is  now  used  almost  exclusively  in  old  works  where  soft 
raw  materials — as  chalk  and  river  mud  (or  occasionally 
gault  clay) — are  readily  available,  but  chiefly  about  the 
Thames  and  Medway.  Suitable  proportions  of  the  materials 
(about  3  of  chalk  to  i  of  mud  by  weight),  with  enough  water 
to  make  about  80  per  cent,  in  all,  are  intimately  mixed  in  a 
wash-mill  to  make  slurry  or  slip.  The  wash-mill  is  a  vessel 
about  15  ft.  in  diameter,  the  contents  of  which  are  agitated 
and  mixed  by  means  of  radial  horizontal  arms  fixed  on  a 
rotating  central  vertical  shaft.  The  prepared  slurry  is  run 
off  through  sieves  (or  gratings)  into  troughs  leading  to  large 
tanks  called  backs.  After  settling  in  the  backs,  the  water 
at  the  top  is  drawn  off,  the  moist  pasty  mass  is  dug  out, 
dried,  and  burned.  The  drying  is  mostly  effected  by  waste 
heat  from  kilns,  but  drying  floors,  heated  by  means  of 
waste  gases  from  coke  ovens,  may  be  used.  This  wet  process 
is  almost  obsolete.  On  the  Continent  the  stiff  slurry  from 
the  backs  is  made  into  bricks,  and  dried  by  a  current  of  hot 
air  in  tunnel  dryers. 

The  plant  for  the  wet  process  costs  much  less  than  that 
for  the  dry,  and  requires  less  labour  and  no  rotary  dryers, 
etc.  The  process  also  is  simple,  the  mixture  kept  perfect, 
and  with  long  kilns  scarcely  requires  more  fuel. 

In  the  semi-wet  or  semi-dry  process,  sometimes  termed  the 
Goreham  process,  the  mixing  and  grinding  usually  take 
place  in  wash-mills  like  those  used  in  the  wet  process,  but 
only  about  40  to  50  per  cent,  of  water  is  present,  forming 
thick  slurry.  When  the  materials  are  hard,  they  are  as  a 


142  SILICA   AND   THE  SILICATES 

rule  ground  in  a  wet  ball-mill  or  a  wet  edge-runner. 
Thorough  grinding  and  mixing  are  extremely  important. 
To  ensure  extremely  fine  grinding  of  the  slurry,  it  must  be 
further  reduced  in  other  mills,  or  by  passing  through  a 
series  of  wash-mills,  or  in  a  special  wash-mill — known  as 
Clarke's  mill —  in  which  a  rotating  iron  plate  carries  brushes  ; 
the  centrifugal  action  of  the  brushes  casts  the  slurry  (which 
enters  the  Clarke's  mill  through  a  perforated  iron  plate) 


FIG.  3.—"  Hecla  "  disc  crusher.     (Hadfields,  Ltd.) 

against  the  outer  wall  of  the  tank,  where  the  finer  part  passes 
through  sieves,  and  the  coarser  portions  are  swept  away 
by  the  brushes  and  sent  back  to  the  grinding  mills  through 
an  outlet  near  the  bottom  of  the  tank.  The  slurry  is  finally 
collected  in  large  tanks  called  mixers,  where  it  is  subjected 
to  continuous  agitation  by  mechanical  stirrers  or  compressed 
air.  The  slurry  is  tested,  and  any  necessary  adjustment  made 
by  adding  slurry  of  clay  or  chalk.  The  slurry  is  pumped 
from  the  mixers  to  the  kiln,  or  to  the  drying  floors  if  briquettes 


LIME,   MORTAR,   AND  CEMENT 


143 


are  to  be  made.     On  the  Continent,  briquettes  are  generally 
dried  in  tunnel  dryers.     This  process  is  termed  in  America 
the  wet  process,  the  old  wet  process  not  being  known  there. 
The  dry  process  is  used  mostly  for  hard,  raw  materials, 
though    soft    materials    can    be    treated    similarly.     Hard 


FIG.  4.— Allen's  Stag  tube-mill.     (Edgar  Allen  &  Co.,  Ltd.) 

materials  are  first  crushed,  usually  in  jaw-crushers  (in  this 
country)  or  in  gyratory  crushers  (in  the  United  States). 
Wet,  soft  materials  are  crushed  in  rotary  crushers.  The 
crushed  material  is  dried,  generally  in  rotary  dryers,  and 


FIG.  5. — Allen's  Stag  tube-mill  (sectional  view). 

then  ground,  usually  in  a  ball-mill  and  a  tube-mill,  but 
sometimes  in  one  operation,  as  in  a  Griffin  mill.  The 
limestone  may  be  burned  first,  the  lime  mixed  with  clay,  and 
the  mixture  burned  ;  the  grinding  of  the  hard  limestone  is 
thus  obviated  at  the  expense  of  fuel  for  the  extra  burning. 


144  SILICA   AND  THE  SILICATES 

Ball-mills  are  very  good  for  preliminary  grinding,  but 


FIG.  6. — Sturtevant  "  open-door  "  ring/oll-mill  for  grinding. 
(Sturtevant  Engineering  Co.,  Ltd.) 


FIG.  7. — Sturtevant  "  open-door  "  ring  roll-mill, 
not  so  well  adapted  for  producing  fine  flour.    They  consist 


LIME,   MORTAR,   AND   CEMENT  145 

of  a  circular  drum  which  can  be  rotated  about  a  horizontal 
axis,  and  is  charged  with  forged  steel  balls.  The  material 
to  be  ground  is  fed  into  the  drum,  and  is  crushed  by  the 
action  of  the  balls.  The  details  vary  in  the  mills  of  different 
makers.  Edge-runners  are  in  some  cases  used  for  dry 
grinding  instead  of  ball-mills,  and  must  either  be  followed 
by  a  tube-mill  for  finishing,  or  used  along  with  sieves  or 
air  separators.  The  preliminary  grinding  is  sometimes 
done  in  short  tube-mills  or  pre-grit  mills,  charged  with  steel 
balls,  or  in  other  special  forms  of  grinder. 

Tube-mills  are  used  as  finishing  mills  for  the  fine  grinding 
of  the  grit  from  the  preliminary  grinders  (ball-mills,  etc.). 
They  differ  much  in  details,  but  consist  of  a  steel  tube 
which  can  rotate  about  a  horizontal  axis,  the  cylinder  being 
lined  with  chilled  iron  plates  or  quartzite  blocks,  and  about 
half  filled  with  flint  pebbles,  arranged  so  that  the  larger 
pebbles  are  near  the  inlet  and  the  smaller  ones  near  the 
outlet,  where  the  ground  material  passes  out  through 
grids. 

Compound  mills,  comprising  a  ball-mill  and  a  tube-mill, 
are  sometimes  used.  Centrifugal  roller-mills  with  sieves, 
such  as  the  Griffin  mill,  are  sometimes  used  for  grinding  the 
raw  materials  in  one  operation.  In  other  types  of  mill, 
balls  driven  by  propelling  lugs  take  the  place  of  the  rollers. 

Raw  Flour,  Raw  Meal,  or  Compo,  are  names  applied 
to  the  ground  product  from  finishing  mills.  It  is  tested  for 
fineness  (90  to  95  per  cent. — say  92  per  cent. — should  pass 
through  a  sieve  with  180  meshes  per  linear  inch),  and  the 
proportion  of  calcium  carbonate  is  ascertained  so  that  any 
necessary  change  may  be  made.  When  shaft  kilns  are  used 
for  the  burning,  the  compo  is  stored  in  mixing  silos  with 
constant  stirring,  but  for  rotary  kilns  it  is  taken  at  once  to 
the  small  bins  from  which  the  kilns  are  charged.  The  compo 
from  the  silos  is  mixed  with  a  little  water  when  required, 
and  made  into  rough  bricks  which  are  dried  by  waste  heat, 
or  in  tunnel  dryers.  A  little  lime  is  added  to  the  compo 
sometimes,  to  increase  its  binding  power,  and  sometimes 
bricks  are  made  of  mixed  compo  and  fuel. 

c.  10 


146  SILICA   AND   THE  SILICATES 

Some  works  combine  the  wet  and  dry  process,  by  making 
dry  flour  from  hard  materials  and  wet  slurry  from  soft 
materials. 


FIG.  8.— The  Griffin  mill. 


The  two  are  then  pugged  in  suitable  proportions  and 
made  into  briquettes. 

In  the  burning  the  raw  materials  are  heated  gradually 


LIME,  MORTAR,  AND  CEMENT  147 

to  about  1400°  C.  In  this  operation,  water  is  first  expelled, 
then  carbon  dioxide  from  carbonates,  and  eventually  the 
free  lime  combines  with  silica,  alumina,  and  iron  oxide. 

The  kilns  for  burning  cement  are  either  rotary  kilns,  or 
stationary  or  vertical  kilns,  and  the  latter  are  intermittent 
or  continuous  according  as  the  burned  clinker  is  discharged 
before  adding  fresh  material,  or  fuel  and  raw  material  are 
fed  into  the  kiln  as  the  burned  product  is  gradually  removed. 
The  rotary  kiln  is  always  continuous  in  its  action.  Inter- 
mittent kilns  are  now  in  use  only  in  some  old  and  small 
works.  Stationary  continuous  kilns  are  much  used  for 
burning  cement,  but  the  rotary  kiln  is  the  most  generally 
used,  especially  in  America.  The  kilns  used  include  bottle 
kilns  open  at  the  top,  stage  kilns  like  the  Dietzsch  kiln, 
other  shaft  kilns,  chamber  kilns,  the  Hoffmann  or  ring 
kiln,  etc.  The  clinker  from  an  ordinary  fixed  kiln  may 
contain  5  to  15  per  cent,  of  underburned  material. 

The  rotary  kiln  furnishes  a  striking  example,  of  an 
English  invention  which,  after  being  used  for  a  time,  was 
given  up,  and  later  was  perfected  elsewhere  and  made 
completely  successful.  Crampton  patented  a  rotary  kiln 
as  early  as  1877,  but  Ransome  in  1885  patented  the  first 
practical  rotary  kiln,  which  consisted  of  an  iron  cylinder 
25  ft.  long,  and  5  ft.  in  diameter,  slightly  inclined,  supported 
on  rollers  and  rotated  by  means  of  worm  gearing.  The 
cylinder  was  lined  with  firebricks,  longitudinal  ribs  being 
formed  by  setting  every  fourth  row  of  bricks  on  edge  (in- 
stead of  flat).  The  raw  materials,  dried  and  ground,  entered 
at  the  higher  end  of  the  cylinder,  were  gradually  raised  to 
the  clinkering  temperature  as  they  descended  slowly,  and 
eventually  the  clinker  passed  out  at  the  lower  end.  The 
fuel  (producer  gas)  was  burned  at  the  lower  end  of  the 
cylinder,  where  it  entered.  Ransome  claimed  that  clinker 
calcined  in  this  way  would  not  need  crushing  before  grinding, 
but  the  highly-heated  material  always  caked  and  yielded 
clinker  in  small  but  very  hard  lumps.  English  cement 
manufacturers,  after  a  time,  abandoned  the  rotary  kiln, 
including  the  improvements  of  Stokes.  After  some  years 


148  SILICA   AND   THE  SILICATES 

it  was  tried  in  the  United  States,  and  soon  proved  successful 


in  the  hands  of  Hurry  and  H.  J.  Seaman  (the  latter  an 


LIME,  MORTAR,   AND   CEMENT  149 

Englishman),  and  others.  A  longer  cylinder  was  found  to 
give  much  better  results,  and  as  fuel,  instead  of  producer 
gas  or  oil,  finely -ground  coal  dust  was  adopted,  and  is  now 
in  general  use  for  the  purpose.  Modern  rotary  kilns  are 
much  larger,  seldom  less  than  100  ft.  long,  and  sometimes 
more  than  twice  that  length ;  the  increased  length  gives 
great  economy  of  fuel. 

The  cylinders  are  usually  constructed  of  riveted  steel 
plates,  and  lined  with  bricks — ordinary  building  bricks  at 
the  cooler  higher  end,  common  firebricks  about  the  middle, 
and  special  aluminous  bricks  at  the  hot  lower  end.  A 
concrete  lining  is  used  in  many  cement  works,  the  concrete 
being  produced  from  a  mixture  of  hard-burned  clinker 
(small  enough  to  pass  easily  through  a  half -inch  mesh  sieve), 
with  half  to  three-quarters  as  much  cement.  The  sides  of 
the  cylinder  may  be  parallel,  or  they  may  taper  upwards, 
or  the  lower  part  may  be  made  larger  in  diameter  than  the 
rest.  This  last  arrangement  has  been  found  to  give  a 
greater  output  than  the  ordinary  parallel-sided  form,  and 
also  to  decrease  the  tendency  of  the  clinker  to  stick  to  the 
lining. 

The  cylinder  is  supported  on  several  pairs  of  roller 
bearings,  and  is  caused  to  rotate  by  a  split  toothed-wheel 
worked  by  a  worm-wheel  and  reduction  gearing.  Generally 
there  is  a  three-speed  gear  to  regulate  the  velocity  of  rotation, 
which  varies  as  a  rule  from  30  to  60  revolutions  per  hour, 
according  to  slope  of  kiln,  character  of  the  raw  materials, 
and  heating  conditions  in  the  kiln. 

The  upper  end  of  the  cylinder  communicates  through  a 
flue  with  a  chimney,  or,  where  several  kilns  are  worked 
together,  they  may  be  connected  by  dampered  flues  with 
a  single  large  chimney.  In  all  cases,  the  kiln  gases,  heavily 
charged  with  dust,  are  passed  through  a  dust  chamber 
before  reaching  the  chimney.  The  deposited  dust  is  re- 
moved either  continuously  or  at  intervals. 

The  well-dried,  finely-ground  bituminous  coal  mostly 
used  as  fuel  has  to  be  used  as  soon  as  it  is  ground,  being  too 
explosive  to  be  stored.  From  hoppers  above  the  lower 


150  SILICA   AND  THE  SILICATES 

end  of  the  kiln  it  is  blown  into  the  cylinder  (by  means  of  a 
fan  or  compressed  air,  etc.)  and  at  once  becomes  inflamed. 

The  clinker  is  discharged  as  little  balls  (ranging  from  the 
size  of  a  pea  to  that  of  a  walnut)  into  the  cooler,  which  in 
Europe  (including  England)  is  usually  an  inclined  rotating 
cylinder,  shorter  and  smaller  than  the  kiln,  but  operated  in 
the  same  manner.  While  the  clinker  is  cooled,  the  air  is 
heated  and  passes  on  to  the  kiln.  In  America  vertical 
coolers  of  various  types  are  preferred.  Several  such  coolers 
are  sometimes  used  instead  of  only  one. 

Clinker  can  be  stored  with  less  risk  of  deterioration 
than  ground  cement,  hence  it  is  often  ground  only  as  required. 
Clinker  from  rotary  kilns  is  often  stored  for  a  few  weeks  so 
that  it  may  become  softened,  but  the  result  depends  on 
composition  and  degree  of  burning  ;  clinkers  low  in  lime 
easily  fall  to  powder,  and  high  alumina  clinkers  are  more 
liable  to  crumble  than  more  silicious  clinkers.  The  rotary 
kiln  not  only  saves  much  labour,  but  gives  a  large  output — 
200  to  400  tons  per  kiln  per  week — and  has  other  advantages. 

A  kind  of  blast  furnace  has  been  used  with  some  success 
for  burning  Portland  cement,  and  in  spite  of  commercial 
difficulties  due  to  the  high  temperature  (about  1500°  C.) 
necessary  for  sintering,  the  process  has  merits  of  its  own 
which  may  eventually  bring  it  into  prominence. 

Clinker  properly  burned  is  very  dark- brown,  or  greenish- 
black,  hard,  but  rather  porous.  Under-burned  clinker  (half- 
burned,  slack,  or  pink  as  it  is  called)  is  softer  and  mostly 
greenish-grey,  but  sometimes  yellow,  pink,  reddish,  or 
purple.  Over-burned  clinker  is  hard,  non-porous,  and 
generally  bluish-black,  presenting  a  metallic  appearance. 

The  grinding  of  cement  clinker  is  done  most  frequently 
in  a  ball-mill  and  tube-mill  successively,  but  sometimes  in  a 
Griffin  mill  or  some  other  kind.  Clinker  from  vertical 
kilns  is  first  crushed  before  grinding.  The  small  size  of  the 
lumps  of  clinker  from  rotary  kilns  renders  preliminary 
crushing  unnecessary  in  general,  though  their  great  hardness 
makes  them  more  difficult  to  grind.  Moreover,  special 
treatment  is  necessary  to  regulate  the  time  of  setting  of 


LIME,  MORTAR,  AND  CEMENT 

cement  from  rotary  kilns,  as  the  ground  cement  sets  almost 
immediately  if  the  clinker  is  merely  ground  as  it  comes  from 
the  coolers  and  then  used.  Even  the  clinker  from  vertical 
kilns  has  sometimes  to  receive  special  treatment  for  the 
same  purpose.  The  setting  time  is  mostly  regulated  by 
adding  a  little  gypsum  or  plaster  to  the  clinker,  either 
before  or  during  the  grinding  ;  sometimes  moistening  the 
clinker  with  water  will  have  the  same  effect,  or  both  may 


FIG."  10. — Ball-mill  for  dry  grinding. 

be  used  together ;  another  method  is  to  pass  steam  into 
the  tube-mill  (in  which  the  grinding  is  being  finished)  through 
the  hollow  trunnion  at  the  feed  end,  the  setting  time  of  the 
cement  depending  on  the  pressure  of  the  steam. 

The  finer  material  is  separated  from  the  coarser  particles 
by  means  of  sieves  or  air  separators,  a  regulated  current  of 
air  effecting  the  separation  in  the  latter. 

The  finished  cement  is  placed  in  storage  bins  or  silos, 
and  eventually  packed  into  sacks  or  casks. 


152  SILICA   AND  THE  SILICATES 

Much  dust  is  produced  at  various  stages  in  the  manu- 
facture of  cement,  and  this  is  separated  from  gases  at  a 
high  temperature  in  expansion  chambers ;  dust  is  collected 
in  this  way  from  the  air,  from  dryers,  kilns,  and  coolers. 
Such  dust  chambers  are  also  suitable  for  collecting  dust 
from  crushing  and  grinding  mills,  filters  being  also  used  ; 
instead  of  niters  are  sometimes  employed  collectors  which 
depend  on  centrifugal  force  for  their  effectiveness,  but 
in  most  cases  they  do  not  answer  so  well  as  niters.  The 
Beth  dust  collector  is,  perhaps,  the  best-known  device  ;  the 
air  charged  with  dust  is  drawn  through  cylindrical  sieves 
made  of  textile  fabric,  and  the  dust  adheres  to  the  inside 
of  the  sleeves,  which  are  automatically  cleaned  from  the 
deposited  dust. 

Well-prepared  commercial  Portland  cement  should  be 
bluish-grey,  and  have  a  specific  gravity  of  3*10  at  least.  A 
brownish  tinge  indicates  either  excess  of  clay,  unsuitability 
of  the  clay  used,  or  under-burning  of  the  cement.  As 
D.  B.  Butler  has  shown  that  only  very  fine  particles  have 
useful  cementing  properties,  the  cement  should  be  rery 
finely  ground.  The  modern  tendency  is  towards  increased 
fineness  of  grinding,  and  the  proportion  left  on  a  sieve  with 
180  meshes  to  the  linear  inch  rarely  amounts  to  10  per  cent. 
The  setting  time  varies  to  such  an  extent  that  cements  are 
classed  in  this  country  as  quick-setting,  medium-setting,  and 
slow-setting  cements. 

The  tensile  strength  of  neat  cement  after  seven  days  is 
in  most  cases  from  500  to  700  Ib.  to  the  square  inch,  and  the 
crushing  strength  about  ten  times  as  great  as  a  rule  ;  both 
go  on  increasing  for  a  year  or  more,  and  both  decrease  in 
proportion  to  the  amount  of  sand  mixed  with  the  cement. 

The  composition  of  typical  Portland  cements  varies  only 
slightly,  though  it  may  be  made  from  different  raw  materials. 
The  essential  components  are  lime,  silica,  and  alumina,  but 
oxides  or  iron,  magnesia,  and  alkalies  occur  in  smaller 
amounts,  and  a  little  sulphur  is  also  usually  present.  It  is 
only  in  old  or  badly  burned  cements  that  carbon  dioxide 
and  water  occur  to  any  notable  extent. 


LIME,   MORTAR,   AND  CEMENT  153 

In  the  British  Standard  Specification  (1915)  for  Portland 
cement,  it  is  laid  down  that  the  ratio  of  the  chemical  equiva- 
lent of  lime  to  the  total  equivalents  of  silica  and  alumina — 
that  is,  the  ratio  of  n :  (p+r)  in  wCaO  :  (pSiO^+rAl2O3) 
must  neither  exceed  2*85  nor  be  less  than  2'0. 

Ordinary  free  lime  probably  never  occurs  in  modern 
Portland  cement.  The  calcium  hydroxide  formed  on 
hydration  of  tricalcium  silicate  would  in  the  presence  of 
trass  or  other  suitable  silicious  material  form  a  pozzuolanic 
cement.  The  addition  of  trass  to  Portland  cement  for  this 
purpose  (advocated  by  W.  Michaelis)  certainly  increases 
the  strength,  but  has  not  come  into  general  use. 

Magnesia  is  nearly  always  present  in  small  amounts, 
combining  with  silica  and  alumina  to  form  compounds 
resembling  the  calcium  silicates  and  aluminates.  These 
magnesium  compounds  hydrate  much  more  slowly  than  the 
corresponding  calcium  compounds,  and  their  presence 
would  tend  to  cause  disruption  after  setting.  Messrs. 
Newberry  consider  that  magnesium  silicates  and  aluminates 
have  no  hydraulic  properties.  In  the  British  Standard 
Specification  (1915)  the  magnesia  in  Portland  cement  is 
limited  to  3  per  cent.  According  to  tests  made  by  the 
U.S.  Bureau  of  Standards,  increased  magnesia  was  asso- 
ciated with  a  reduction  in  the  clinkering  temperature  of  the 
cements,  and  also  (as  shown  by  microscopic  examination) 
with  increased  size  of  crystals,  and  appearance  of  new 
constituents— monticellite  and  spinel.  Below  8  per  cent,  of 
magnesia,  the  physical  properties  of  the  cement  were  not 
seriously  affected,  and  the  mechanical  strength  of  neat  and 
mortar  specimens  and  also  of  concrete,  were  comparable  to 
those  obtained  with  cements  having  normal  composition. 
With  more  than  8  per  cent,  magnesia  the  early  strength  was 
less  (though  initial  setting  was  quicker),  but  the  gain  was 
consistent  with  age. 

Iron  oxides  are  the  chief  colouring  agents  in  Portland 
cement,  which  in  the  absence  of  colouring  impurities  is 
almost  as  white  as  snow.  At  the  clinkering  temperature 
iron  |oxides  combine  with  lime  to  form  ferrites,  compounds 


154  SILICA   AND   THE  SILICATES 

which,  according  to  Ive  Chatelier,  slake  and  decompose  in 
contact  with  water,  but  do  not  set.  Sulphides  (when  present) 
act  on  the  iron  compounds  to  form  ferrous  sulphide,  which 
later  becomes  oxidized  to  ferrous  oxide  and  eventually  to 
ferric  oxide. 

Alkalies  in  Portland  cement  include  potash  and  soda  in 
the  form  of  silicates.  They  are  derived  chiefly  from  the 
clays  and  shales,  but  rarely  amount  to  i  per  cent.  As  some 
alkaline  silicates  dissolve  in  water,  they  may  serve  a  useful 
purpose  in  carrying  silicic  acid  during  the  setting  of  the 
cement. 

Sulphur  in  Portland  cement  originates  from  calcium 
sulphate  and  iron  pyrites  in  clay  and  shales,  from  the  fuel, 
and  from  the  calcium  sulphate  usually  added  to  retard 
the  setting  time  of  the  calcined  cement.  Small  quantities 
of  calcium  sulphate  (i  to  2  per  cent,  of  either  gypsum  or 
plaster),  and  also  up  to  2  per  cent,  of  water,  added  after 
calcination,  are  beneficial  for  regulating  the  setting  time. 
L,arger  quantities  promote  the  formation  of  the  highly 
expansive  calcium  sulpho-aluminate,  and  so  are  harmful ; 
calcium  sulphate  is  also  softer  than  cement,  and  liable  to  be 
washed  out  by  water.  The  maximum  of  sulphur  trioxide 
allowed  by  the  British  Standard  Specification  (1915)  is 
275  per  cent. 

Typical  analyses  of  good  Portland  cements  show  60  to 
63  per  cent,  of  lime,  20  to  25  silica,  6  to  8  alumina,  2  to  5 
ferric  oxide,  with  about  i  per  cent,  (not  more  than  2) 
each  of  magnesia,  sulphur  trioxide,  loss  of  ignition,  and 
alkalies.  The  silica  and  alumina  should  saturate  all  the 
lime  present. 

CONSTITUTION  OF  PORTLAND  CEMENT. 

Though  the  composition  of  good  cement  varies  only 
within  narrow  limits,  the  properties  depend  more  on  the 
way  in  which  the  constituents  are  combined  than  on  the 
proportions.  Numerous  investigations  have  been  made 
on  the  constitution  of  cement  and  on  the  processes  of  setting 


LIME,   MORTAR,   AND   CEMENT  155 

and  hardening,  including  analyses  of  cements  and  careful 
study  of  their  chemical  and  physical  properties,  microscopic 
examination  of  clinker  and  of  hardened  cement,  study  of 
reactions  occurring  during  the  formation  and  cooling  of 
cement  clinker  (more  particularly  changes  in  electric  con- 
ductivity), and  synthetical  studies  of  possible  components 
of  cements,  with  comparison  of  the  properties  of  such 
synthetical  products  with  those  of  cements. 

Analysis  of  cements  gives  little  information  as  regards 
constitution,  though  it  serves  to  show  the  compositions  of 
the  best  products.  Study  of  chemical  properties  also  throws 
little  light  on  the  subject. 

Microscopic  investigations  have  yielded  important  results, 
the  microscope  being  used  in  two  different  ways.  In  one 
method,  a  thin  section  of  the  material  is  placed  under  a 
petrological  microscope  for  examination  by  transmitted 
light.  In  the  second  method,  a  small  piece  of  clinker  or 
other  material  has  one  side  only  ground,  polished,  and 
etched,  and  then  examined  by  reflected  light.  This  second 
method  is  applicable  to  hardened  cement,  which  is  not  hard 
enough  for  the  preparation  of  the  thin  sections  needed  for 
the  first  method.  A  surface  of  cement  clinker  etched  with 
a  i  per  cent,  solution  of  hydrochloric  acid  in  alcohol  is  seen 
under  the  microscope  to  consist  of  crystalline  grains  em- 
bedded in  a  matrix,  the  only  apparent  difference  between 
the  clinker  from  vertical  kilns  and  that  from  rotary  kilns 
being  that  the  latter  is  generally  much  finer  grained.  I^e 
Chatelier,  and  also  Tornebohm,  distinguished  four  kinds  of 
crystals,  which  were  termed  alite,  Mite,  celite,  and  /elite 
respectively. 

Alite  seems  to  be  the  chief  component  of  Portland 
cement  clinker,  and  forms  the  crystalline  grains  noticed  in 
etched  clinker.  It  occurs  as  nearly  colourless  crystals,  and 
the  amount  increases  with  the  proportion  of  lime  in  the 
clinker.  Pure  alite  has  been  found  to  consist  of  lime,  silica, 
alumina,  magnesia,  and  ferrous  oxide. 

Alite  is  decomposed  and  hydrated  by  water. 

Belite  is  not  invariably  present  in  cement  clinker,  but 


156  SILICA   AND   THE  SILICATES 

generally  occurs  in  clinker  poor  in  lime.  It  is  not  so  light- 
coloured  as  alite,  and  usually  shows  fine  striations. 

Celite  is  the  matrix  between  the  alite  grains,  and  must 
have  been  melted  at  the  clinkering  temperature.  Alite 
and  celite  are  the  only  essential  components  of  Portland 
cement,  and  the  amount  of  celite  increases  with  the  pro- 
portion of  ferric  oxide.  The  colour  of  celite  is  a  deep 
brownish-orange,  indicating  the  much  larger  proportion  of 
iron  present  in  it  than  in  alite.  Celite  is  only  slightly 
affected  by  water. 

Felite  seldom  occurs  in  cement  clinker,  but  it  occurs  in 
blast-furnace  slag.  H.  Kappen  considers  that  high  temper- 
ature calcination  promotes  the  formation  of  f elite  in  clinker. 
Felite  and  belite  seem  to  be  two  varieties  of  the  same  solid 
solution. 

C.  Richardson  compares  cement  clinker  to  an  alloy.  In 
pure  cement  the  alite  is  a  solid  solution  of  tricalcium  silicate 
in  tricalcium  aluminate,  and  the  celite  is  a  solid  solution  of 
dicalcium  aluminate  in  dicalcium  silicate,  whilst  in  com- 
mercial Portland  cement  occur  compounds  of  ferric  oxide 
and  lime  corresponding  to  the  aluminates ;  magnesia, 
alkalies,  and  sulphur  trioxide  are  considered  to  be  non- 
essential  components. 

Keiserman  investigated  the  effects  of  certain  dyes  on 
the  chief  components  of  cement,  and  found  that  patent 
blue  answered  best  for  detecting  alumina,  anthropurpurin 
(in  alcohol)  for  free  lime,  and  methylene  blue  for  silica — 
acetic  acid  solution  for  combined  silica,  and  neutral  solution 
for  free  amorphous  silica.  He  concluded  from  his  exami- 
nation of  cement  in  this  way  under  the  microscope  that 
Portland  cement  clinker  is  mainly  composed  of  dicalcium 
silicate  and  tricalcium  aluminate,  probably  without  alumino- 
silicates ;  the  proportions  of  these  components  in  cement 
clinker  correspond  to  the  formula  4(2CaO.SiO2)  +3CaO.Al2Oc. 

Ditter  and  Jesser  studied,  with  the  aid  of  the  microscope, 
the  changes  occurring  during  the  sintering  of  Portland 
cement,  and  found  that  the  electrical  conductivity  gradually 
increased  up  to  1375°  C.,  at  which  point  the  process  was 


LIME,   MORTAR,   AND  CEMENT  157 

endothermic.  Between  1425°  C.  and  1450°  C.  the  reaction 
was  strongly  exothermic,  and  at  1500°  C.  it  became  endo- 
thermic again.  At  about  1375°  C.  the  sintering  of  the 
particles  was  observable  by  means  of  the  heat- microscope 
and  between  1425°  and  1450°  C.  crystals  of  alite  separated 
suddenly,  with  a  little  celite. 

Janecke  investigated  the  quaternary  system  CaO— SiO2 
— A12O3— Fe2O3,  and  concluded  that  ferric  oxide  only  occurs 
combined  with  lime,  and  does  not  form  solid  solutions. 
He  also  concluded  that  a  single  ternary  compound  was 
produced,  corresponding  to  the  formula  8CaO.2SiO2.Al2O3, 
which  he  considered  identical  with  alite,  thus  accounting  for 
the  necessity  of  alumina  being  present  in  Portland  cement. 
He  further  concluded  that  belite,  celite,  and  felite  have 
compositions  represented  by  2CaO.vSiO2,  3CaO.Fe2O3,  and 
CaO  respectively.  A  little  later,  Janecke  and  Schumann 
arrived  at  the  conclusion  that  the  compound  8CaO.2SiO2.- 
A12O3  constituted  the  chief  constituent  of  Portland  cement 
clinker.  Rankin  and  Wright,  on  the  other  hand,  contended 
that  Janecke's  supposed  compound  is  a  mixture  of  calcium 
silicates  and  aluminates,  the  components  at  ordinary 
temperatures  being  tricalcium  silicate,  dicalcium  silicate, 
and  tricalcium  aluminate. 

W.  and  D.  Asch  assume  the  existence  of  a  number  of 
very  similar  cements  consisting  of  calcium  salts  of  complex 
alumina-silicic  acids,  which  occur  as  various  isomerides 
and  polymerides,  to  which  imposing  constitutional  formulae 
are  ascribed.  These  are  highly  speculative,  and,  though 
very  ingenious,  cannot  be  regarded  as  fairly  established. 

lye  Chatelier,  as  a  result  of  synthetic  studies  (1882  to 
1887) — using  definite  proportions  of  pure  lime,  silica, 
alumina,  etc. — concluded  that  the  hardening  of  Portland 
cement  is  due  to  tricalcium  silicate  (3CaO.SiO2),  and  that 
the  initial  setting  is  due  chiefly  to  tricalcium  aluminate 
(3CaO.Al2O3),  but  in  some  degree  also  to  dicalcium  alumi- 
nate (2CaO.Al2O3)  and  monocalcium  aluminate  (CaO.Al2O3), 
as  all  three  aluminates  set  in  water.  Tricalcium  silicate 
cannot  be  produced  by  the  direct  combination  of  lime  and 


158  SILICA   AND   THE  SILICATES 

silica,  but  is  formed  in  the  cement  clinker  by  combination 
of  lime  and  another  silicate.  Dicalcium  silicate  (2CaO.SiO2) 
forms  twinned  crystals,  which  separate  on  cooling  because 
of  unequal  contraction  of  the  opposite  faces,  and  so  give 
rise  to  the  spontaneous  disintegration  of  cement  clinker. 
Monocalcium  silicate  does  not  react  with  water,  and  so 
cannot  affect  the  hardening  of  cement. 

S.  B.  and  W.  B.  Newberry  found  later  (1897)  that 
powdered  tricalcium  aluminate  after  mixing  with  water 
cracked  after  setting  and  disintegrated  entirely  when 
immersed  in  water.  They  also  claimed  to  have  brought 
about  direct  union  of  lime  and  silica  to  form  tricalcium 
silicate.  They  considered  tricalcium  silicate  and  dicalcium 
aluminate  to  be  the  chief  components  of  Portland  cement 
clinker. 

Day,  Shepherd,  and  Wright  (1906)  have  shown  that 
tricalcium  silicate  cannot  be  produced  from  lime  and  silica 
alone,  and  that  the  material  obtained  by  the  Newberrys 
was  composed  of  an  ultimate  intergrowth  of  crystals  of 
calcium  silicate  (calcium  orthosilicate)  and  lime. 

The  elaborate  researches  at  the  Geophysical  laboratory 
(Washington)  have  definitely  established  that  the  essential 
components  of  a  pure  and  well-burned  Portland  cement 
are  dicalcium  silicate  (the  beta  form  only),  tricalcium  silicate, 
and  tricalcium  aluminate.  In  perfectly  burned  clinker 
there  may  be  in  addition  some  free  lime  and  also  the  calcium 
aluminate,  5CaO.3Al2O3  ;  the  latter  may  also  arise  from 
raw  material  low  hi  lime  and  high  in  alumina.  Commercial 
cement  almost  invariably  contains  ferric  oxide,  magnesia, 
potash,  and  soda,  though  none  of  these  appears  to  be  at  all 
essential,  and  free  lime  is  usually  present  in  small  quantities. 
Dicalcium  silicate  is  the  first  silicate  to  be  formed,  and  it  is 
only  with  difficulty  that  this  compound  afterwards  takes 
up  more  lime  to  produce  tricalcium  silicate.  This  latter 
exists  in  well-defined  crystals  in  well-burned  clinker  with 
high  silica,  but  such  clinker  is  exceptional.  The  first 
aluminate  formed  is  5CaO.3Al2O3,  which  later  combines 
with  more  lime  to  form  tricalcium  aluminate.  The  alite  of 


LIME,   MORTAR,   AND  CEMENT  159 

earlier  investigators  was  apparently  a  mixture  of  3CaO.SiO2 
and  3CaO.Al2O3,  and  celite  was  probably  either  2CaO.SiO2 
or  an  intimate  mixture  of  this  and  3CaO.Al2O3  (or 
2CaO.Al2O3  according  to  some  authorities). 

The  setting  and  hardening  of  Portland  cement  involves 
the  formation  of  an  amorphous  hydrated  material  which 
subsequently  partly  crystallizes. 

I^e  Chatelier  explained  the  initial  setting  of  Portland 
cement  as  being  analogous  to  that  of  plaster,  resulting 
mainly  from  the  formation  of  a  supersaturated  solution  of 
tricalcium  aluminate,  which  immediately  deposits  hydrated 
crystals  of  3CaO.Al2O3.i2H2O.  The  solution  can  then 
dissolve  some  more  anhydrous  tricalcium  aluminate,  and 
again  deposits  hydrated  salt,  the  process  of  solution  and 
deposition  proceeding  at  the  same  time  at  neighbouring 
points.  The  later  hardening  is  attributed  to  decomposition 
of  the  tricalcium  silicate,  which  reacts  with  water  to  produce 
hydrated  monocalcium  silicate,  (CaO.SiO2)2.5H2O,  and 
calcium  hydroxide ;  the  former  appears  in  very  small 
needles,  and  the  latter  as  hexagonal  plates. 

The  initial  set  is  probably  due  to  hydration  of  tricalcium 
aluminate  (with  or  without  amorphous  alumina).  Sulpho- 
aluminate  crystals  (a  combination  of  calcium  sulphate  and 
calcium  aluminate)  may  be  formed  at  the  same  time,  and 
any  free  lime  becomes  hydrated.  The  formation  of  these 
compounds  begins  soon  after  the  cement  is  gauged  with 
water.  Within  24  hours  tricalcium  silicate  begins  to  be 
hydrated,  and  after  a  week  or  more  the  amorphous  alumi- 
nate begins  to  crystallize,  and  calcium  disilicate  begins 
to  hydrate.  Calcium  disilicate  is  the  chief  constituent 
of  American  Portland  cements.  The  ideal  cement  should 
perhaps  have  an  excess  of  dicalcium  silicate,  which  would 
give  a  not  too  dense  hydrated  material,  gaining  strength 
later.  L,ess  of  the  tricalcium  silicate  would  give  the  desirable 
early  strength,  and  also  remedy  the  excessive  porosity  of  the 
dicalcium  silicate. 

The  hardness  and  cohesive  strength  of  Portland  cement 
are  due  at  first  to  the  cementing  action  of  the  amorphous 


160  SILICA   AND  THE  SILICATES 

material  produced  by  the  tricalcium  aluminate  and  of 
tricalcium  silicate,  and  the  gradual  increase  in  strength  is 
due  to  further  hydration  of  these  two  compounds,  together 
with  the  hydration  of  dicalcium  silicate.  Tricalcium  silicate 
is  apparently  the  essential  constituent  of  Portland  cement, 
probably  because  gelatinous  silica  is  so  readily  liberated  when 
it  is  mixed  with  water. 

It  may  be  added  that  a  cement  can  be  made  having  all 
the  properties  of  Portland  cement,  and  containing  less  than 
0*5  per  cent,  alumina.  But  such  a  cement  cannot  be 
produced  commercially,  as  it  would  require  too  high  a 
temperature.  In  the  manufacture  of  Portland  cement, 
alumina  is  necessary  for  lowering  the  burning  temperature 
to  such  an  extent  that  a  ternary  system  may  be  formed 
without  too  great  an  expenditure  of  heat,  and  from  which 
the  tricalcium  silicate  (the  active  hardening  constituent) 
may  separate. 

POZZUOI.ANA  CEMENTS. 

Pozzuolanas  are  volcanic  lavas  modified  by  the  action 
of  superheated  steam  and  perhaps  other  gases,  so  that  they 
have  become  powdery,  and  have  also  acquired  hydraulic 
properties.  The  name  is  derived  from  Pozzuoli,  near 
Naples,  where  such  material  was  found  in  early  times. 
Similar  material  occurs  elsewhere  in  Italy,  and  also  in  France 
and  other  places.  The  chief  constituents  are  silica  and 
alumina,  with  smaller  proportions  of  iron  oxides,  lime, 
magnesia,  and  alkalies.  The  silica  can  easily  combine  with 
lime,  when  water  is  present,  to  form  calcium  silicate. 

Pozzuolana  may  be  white,  grey,  brown,  black,  yellow, 
etc.,  and  is  mostly  amorphous,  but  contains  crystalline 
mica,  and  other  minerals.  Sometimes  it  occurs  as  fine 
powder,  sometimes  in  coarser  grains,  sometimes  in  the  form 
of  pumice,  etc.  The  material  is  screened  and  ground. 

Trass  is  a  material  of  somewhat  similar  nature,  found 
in  the  Eifel  district  by  the  Rhine,  and  is  pale  yellow  or  grey. 
It  may  be  described  as  an  altered  volcanic  ash,  and  is 


LIME,  MORTAR,  AND  CEMENT  161 

generally  more  compact  than  ordinary  pozzuolanas,  but  is 
sometimes  porous  like  pumice.  As  regards  composition,  it 
contains  relatively  less  lime  and  more  silica  than  pozzuolana. 

Santorin  earth  is  a  light-grey  volcanic  ash,  from  the 
Grecian  island  Santorin  (Thera),  which  has  similar  characters, 
but  is  more  silicious.  Pumice  and  volcanic  tuff  have  also 
similar  properties. 

Pozzuolanic  materials  have  in  some  cases  resulted 
from  alteration  of  sandstones  and  limestones  by  contact 
with  eruptive  igneous  rocks.  Even  basalts  and  trap  rocks, 
when  crushed  and  ground,  give  similar  results,  but  can 
scarcely  be  used  in  this  way  economically  because  of  their 
greater  hardness. 

Natural  pozzuolanic  materials  are  rarely  used  in  England 
for  making  mortar,  and  not  very  much  abroad,  owing  to 
the  better  qualities  possessed  by  the  natural  and  slag  cements, 
and  especially  by  Portland  cement. 

Pozzuolanic  (or  trass)  cements  or  mortars  are  made  by 
mixing  the  materials  with  slaked  lime  (preferably  in  pasty 
condition  except  when  very  hydraulic  lime  is  used),  the 
pozzuolanic  material  being  generally  mixed  with  sand  or 
other  inert  material  to  reduce  shrinkage  and  prevent 
cracking,  and  also  to  increase  the  porosity,  at  the  same 
time  reducing  the  cost.  Contrary  practice  notwithstanding, 
the  lime  paste  should  be  stiff  for  materials  consisting  of 
hard  and  palpable  particles,  but  thinner  for  fine-grained 
impalpable  materials.  The  best  proportions  in  the  case 
of  common  lime  are  considered  to  be  i  of  lime  (powder)  to 
2j  of  pozzuolana,  to  2  of  trass,  or  to  i  of  sand  with  i  of 
pozzuolana  or  trass. 

The  simplest  pozzuolana  cement  would  be  a  mixture  of 
pure  lime  and  hydrated  silica,  but  the  latter  is  too  costly 
for  use  in  making  cement.  The  same  is  the  case  with 
kieselguhr,  a  natural  active  silica. 

The  name  pozzuolana  cement  or  Victoria  cement,   is 

sometimes  given  to  a  mixture  of  3  parts  of  finely -ground 

slag    (previously  granulated  by  running  into  water)    and 

i  part  by  weight  of  slaked  lime.     If  kept  moist  enough,  the 

c.  ii 


i62  SILICA   AND   THE  SILICATES 

mixture  sets  slowly  and  becomes  fairly  hard  and  strong. 
Frost  acts  destructively  upon  slag  cement  structures. 
Slag  cements  also  have  a  bad  tendency  to  crack,  which  can 
only  be  obviated  by  not  grinding  the  slag  so  fine,  and  they 
are  not  very  strong.  The  initial  hardness  is  not  great,  which 
is  also  the  case  with  all  natural  and  artificial  pozzuolanic 
cements. 

CONCRETE. 

Concrete  (or  beton)  consists  of  a  matrix  of  mortar — 
either  lime  mortar  or  cement  mortar — in  which  are  embedded 
pebbles  or  broken  stones  forming  what  is  called  the  aggregate. 
The  aggregate  may  be  gravel  or  broken  stone,  broken  bricks, 
slag,  clinker,  coke,  and  ashes,  etc.  Rough  angular  pieces 
bond  better  with  the  mortar  than  rounded  pebbles  do. 
When  strength  is  not  essential,  and  a  light  concrete  is 
desirable,  a  porous  substance  (such  as  pumice  or  coke)  is 
used  for  the  aggregate.  Coke  and  ashes  containing  oxidiz- 
able  sulphur  compounds  are  liable  to  undergo  expansion,  and 
thus  to  cause  rupture  of  the  concrete  ;  slags  and  clinker 
also  contain  such  sulphur  compounds,  and  are  therefore 
also  liable  to  expand. 

Careful  grading  of  the  aggregate,  using  pieces  of  various 
sizes,  enables  the  strength  of  the  concrete  to  be  nearly 
doubled.  Some  gravels  need  no  grading,  as  they  contain 
particles  of  proper  sizes  in  correct  proportions.  The  largest 
pieces  of  the  aggregate  should  in  most  cases  pass  through  a 
i -in.  hole,  but  they  may  be  much  larger  for  great  masses  of 
concrete ;  the  smallest  pieces  should  not  pass  through  a 
J-in.  hole,  and  all  smaller  material  should  be  removed 
(preferably  by  washing),  and  the  aggregate  thoroughly 
wetted.  The  proportion  of  spaces  between  the  pieces 
should  not  exceed  45  per  cent.  The  sands  used  should  be 
clean  and  roughly  angular,  and  well  graded — from  about 
I  to  ^4  in. 

A  very  good  concrete  to  set  quickly  in  water  is  made 
from  a  hydraulic  mortar  (consisting  of  hydraulic  lime, 


LIME,   MORTAR,  AND  CEMENT  163 

pozzuolana  or  trass,  and  sand)  with  which  broken  stones 
and  gravel  are  mixed.  A  good  concrete  for  water  work  may 
also  be  made  by  mixing  mortar  (produced  from  3  parts  of  fine 
sand  to  i  of  hydraulic  lime  unslaked)  with  an  equal  quantity, 
or  even  half  as  much  again,  of  broken  stones  or  gravel. 

For  most  purposes,  including  foundations,  Portland 
cement  concrete  may  be  made  from  i  volume  cement, 
2  volumes  sand,  and  4  to  6  volumes  aggregate — that  is, 
from  an  ordinary  mortar  with  aggregate.  Where  weight 
rather  than  strength  is  required,  much  poorer  concrete  is 
sometimes  used.  For  a  watertight  reservoir,  it  would  be 
better  to  use  ij  to  2  volumes  sand,  and  2-J  to  4  volumes 
aggregate.  For  ordinary  operations,  from  20  to  30  per 
cent,  of  water  is  used,  but  much  less  for  quick  setting,  and 
much  more  when  setting  is  to  be  retarded.  When  the 
water  is  sufficient  to  make  the  consistency  like  that  of  cream, 
and  the  particles  of  aggregate  are  very  small,  the  mixture  is 
termed  grout.  Clean  rain  water  is  preferable  when  available. 

In  making  concrete  the  materials  should  be  mixed  dry, 
first  the  sand  and  cement,  then  the  aggregate  added  to  the 
mixture,  and  finally  water  added,  and  the  mixing  completed 
rapidly. 

Reinforced  Concrete,  Ferro-Concrete,  or  Armoured 
Concrete,  as  it  is  variously  termed,  is  a  combination  of 
concrete  with  steel,  which  has  much  greater  strength  than 
the  concrete  by  itself,  and  the  concrete  also  serves  to  protect 
the  steel  against  corrosion,  and  from  the  expansion  and 
contraction  which  might  result  from  great  changes  of 
temperature.  At  ordinary  temperatures  the  coefficients  of 
expansion  of  concrete  and  steel  are  nearly  the  same.  The 
steel  (usually  "  mild  ")  is  used  in  the  form  of  rods  or  bars, 
wire-netting,  etc.,  but  mostly  rods  are  used  to  increase  the 
tensile  strength  where  it  is  most  needed. 

Reinforced  concrete  is  extensively  used  in  buildings, 
bridges,  boats,  railway  sleepers,  telegraph  poles,  gate  posts, 
tiles,  drain  pipes,  etc. 

Mixed  Cements  are  produced  by  mixing  ground  slags 
with  ordinary  Portland  cement,  but  a  good  Portland  cement 


164  SILICA   AND  THE  SILICATES 

cannot  be  improved  in  this  way,  and  the  added  slag  is  to  be 
regarded  as  an  impurity. 


SOME  SPECIAL  CEMENTS. 

American  Natural  Cements  are  made  from  cement 
rock,  a  comparatively  soft  limestone  containing  13  to  35 
per  cent,  of  argillaceous  material  and  some  magnesia. 
Those  with  the  larger  proportions  of  lime  are  called  (in 
America)  natural  Portland  cements,  and  are  calcined  at  a 
higher  temperature  than  those  with  less  lime,  but  they 
contain  10  to  15  per  cent,  less  lime  than  Portland  cement 
does,  and  are  also  more  variable  in  composition ;  a  true 
Portland  cement  is  obtained  by  mixing  them  with  a  suitable 
proportion  of  non- argillaceous  limestone.  Those  with 
smaller  proportions  of  lime  are  calcined  at  a  lower  temper- 
ature, and  resemble  Roman  cement,  but  have  a  larger 
proportion  of  magnesia.  Rosendale  Cement  is  of  the  latter 
type,  and  contains  about  15  to  18  per  cent,  magnesia,  only 
2  to  4  per  cent,  alumina,  and  only  18  to  25  per  cent,  silica. 
Louisville  Cement  resembles  Rosendale  cement,  but 
contains  less  magnesia. 

The  great  drawback  of  natural  cements  is  that  great 
care  is  needed  in  order  to  use  them  successfully  ;  if  such  a 
cement  be  not  used  immediately  after  making,  or  if  too 
little  or  too  much  water  be  added,  it  has  a  great  tendency 
to  solidify  unequally,  giving  rise  to  cracks,  and  adhering 
badly  to  the  materials. 

Cements  only  need  a  little  water  to  work  them  up  to  their 
greatest  strength,  about  one-third  the  volume  of  the  cement 
giving  the  best  result,  according  to  General  Treussart. 
The  more  the  cement  is  turned  over  before  the  setting 
begins,  the  harder  it  becomes.  As  the  setting  begins  in  a 
comparatively  short  time,  the  cement  should  only  be  pre- 
pared in  amounts  which  can  be  used  up  at  once. 

Passow  Cement  is  a  more  recent  product,  made  by 
specially  granulating  blast-furnace  slag  of  suitable  com- 
position, and  then  grinding  finely,  either  alone  or  with  an 


LIME,  MORTAR,  AND  CEMENT  165 

admixture  of  about  10  per  cent,  of  Portland  cement  clinker. 
It  is  not  (like  ordinary  slag  cement)  a  pozzuolanic  cement, 
depending  on  the  interaction  of  granulated  slag  and  lime. 
The  slag  for  Passow  cement  is  granulated  so  as  to  produce 
a  material  which  itself  sets,  and  attains  a  strength  comparable 
to  that  of  Portland  cement.  Passow  cement  has  been 
successfully  made  from  slags  of  different  compositions  in 
Germany,  England,  and  America. 

Aluminate  Cements.— According  to  P.  H.  Bates  (Journal 
of  the  American  Ceramic  Society,  /,  679,  1918),  it  is  possible, 
by  a  method  and  equipment  like  that  used  for  Portland 
cement,  to  make  cements  which  give  in  24  hours 
strengths  as  high  as  those  usually  developed  by  Portland 
cements  in  28  days,  either  as  mortar  or  concrete. 
This  quick-hardening  cement  is  not  quick  setting.  It  is  a 
calcium  aluminate  (preferably  with  55  to  75  per  cent, 
alumina).  The  presence  of  silica  or  iron  oxide  is  not 
desirable,  but  they  may  be  present  up  to  15  per  cent,  if  they 
replace  lime  and  not  alumina.  The  actual  cost  of  manu- 
facture would  not  exceed  that  of  Portland  cement,  but  the 
cost  of  the  alumina  would  preclude  the  use  of  the  product 
except  for  special  purposes.  Addition  of  3  per  cent,  plaster 
markedly  accelerates  the  setting  of  these  aluminate  cements 
(contrary  to  its  action  with  Portland  cement). 

All  the  calcium  aluminates  have  hydraulic  properties 
except  tricalcium  aluminate,  which  happens  to  be  the  only 
aluminate  present  in  Portland  cement  of  normal  composition 
and  burning.  But  in  the  presence  of  hydrated  lime  and 
water  the  tricalcium  aluminate  forms  a  smooth  plastic  paste 
which  hardens  in  air  like  Portland  cement.  In  Portland 
cement,  lime  may  either  occur  originally  uncombined,  or 
it  may  be  liberated  by  water  from  tricalcium  silicate. 

Plastic  Cements  are  adhesives  applied  to  secure  joints 
and  connections  of  a  more  or  less  permanent  character. 
J.  B.  Barnitt  gave  a  fairly  comprehensive  collection  of  them 
in  the  General  Chemical  Bulletin  for  February,  1917,  which 
was  reproduced  in  the  Journal  of  the  Society  of  Chemical 
Industry,  36,  443,  1917.  They  consist  of  solids  dissolved 


166  SILICA   AND   THE  SILICATES 

or  suspended  in  suitable  vehicles,  the  solids  being  little 
or  not  at  all  affected  by  the  gases  or  liquids  about  them. 
The  following  include  such  as  do  or  may  contain  silicious 
materials  : — 

Hydraulic  Cement  is  used  either  alone  or  with  sand, 
asbestos,  etc.,  and  is  specially  resistant  to  nitric  acid. 
Plaster  of  Paris  is  often  used  alone  as  a  paste  for  joints  on 
gas  and  wood  distillation  retorts  and  similar  places  where 
quick  setting  is  needed.  A  fibrous  material  such  as  asbestos 
is  often  mixed  with  it  to  give  strength ;  stone,  glass,  and 
various  mineral  substances  are  used  as  fillers.  Plaster  and 
water,  and  wet  plaster  with  asbestos,  straw,  plush  trimmings, 
hair,  or  broken  stone,  are  particularly  suitable  cements  for 
oil  vapours  and  hydrocarbon  gases.  Clay  is  often  present 
in  plastic  cements  as  a  filler.  Clay  and  linseed  oil  are  suit- 
able for  steam,  the  same  two  materials  with  fireclay  added 
for  chlorine,  and  clay  and  molasses  for  oil  vapours.  Prepara- 
tions of  linseed  oil  include  China  clay  and  linseed  oil,  lime 
and  linseed  oil  (forming  putty),  and  red  or  white  lead  and 
linseed  oil ;  these  mixtures  become  very  strong  when  set, 
and  are  best  diluted  with  powdered  glass,  clay  or  graphite. 
Silicate  Cements  for  hot  oil  vapours  include  a  paste  of 
sodium  silicate  and  asbestos ;  for  gaskets  for  superheated 
steam  retorts,  furnaces,  etc.,  sodium  silicate  and  glass,  or 
sodium  silicate  50,  asbestos  15,  and  slaked  lime  10  ;  metal 
cement  consists  of  equal  parts  of  sodium  silicate  with  oxide 
of  zinc,  lead,  or  iron,  or  with  mixtures  of  these  oxides. 

For  insertion  of  glass  tubes  in  brass  or  iron  for  high 
temperatures  is  used  a  mixture  of  alumina  I,  sand  4,  slaked 
lime  i,  and  borax  ^,  with  water.  As  a  coating  for  glass- 
ware to  protect  from  injury  by  direct  flame  is  used  a  mixture 
of  fireclay  and  plumbago  made  into  a  paste  with  water. 

Of  other  adhesive  cements  the  following  may  be 
mentioned.  A  strong  cement  for  alabaster  and  marble, 
which  sets  in  a  day,  is  obtained  by  mixing  12  parts  Portland 
cement,  8  fine  sand,  and  i  infusorial  earth,  and  making  into 
a  thick  paste  with  sodium  silicate  (water-glass)  ;  no  heating 
is  necessary  in  applying  it. 


LIME,   MORTAR,   AND   CEMENT  167 

Cutlers'  Cement  for  fixing  knife -blades  in  their  hafts, 
is  made  of  equal  parts  of  brick-dust  and  melted  rosin,  or 
of  4  parts  rosin  with  i  each  of  beeswax  and  brick-dust. 
For  covering  bottle  corks,  a  mixture  of  pitch,  brick-dust, 
and  rosin  is  used.  A  cheap  cement,  sometimes  used  to  fix 
iron  rails  in  stone-work,  is  melted  sulphur  (brimstone),  or 
sulphur  and  brick-dust. 

STATISTICS. 

Cement  manufacture  is  quite  a  modern  industry.  Some 
idea  of  its  progress  may  be  gathered  from  the  following 
approximate  figures  relating  to  the  production  of  Portland 
cement : — 

1890  1907 

Great  Britain      . .     1,000,000  tons       2,750,000  tons 
Germany..          ..     1,500,000     ,,          5,000,000     ,, 
United  States     . .          50,000     ,,          8,000,000     ,, 

Three  years  later  the  total  production  had  reached 
something  like  25,000,000  tons.  The  future  demand  for 
constructional  purposes  will  be  enormous. 

LITERATURE. 

"  The  Portland  Cement  Industry,"  W.  A.  Brown.     London,  1916. 

"  Limes  and  Cements,"  E.  A.  Dancaster.     London,  1916. 

"  Lectures  on  Cement,"  B.  Blount.     London,  1912. 

Encyclopedia  Britannica,  nth  edition,  vol.  v.,  Article  by  B.  Blount. 
Cambridge,  1910. 

Thorpe's  "  Dictionary  of  Applied  Chemistry,"  vol.  i.  Article  by 
B.  Blount.  London,  1912. 

"  Modern  Manufacture  of  Portland  Cement,"  P.  C.  H.  West.  London, 
1910. 

"  Portland  Cement,"  D.  B.  Butler.     London,  1905. 

"  Calcareous  Cements,"  G.  R.  Redgrave  and  C.  Spackman.  London, 
1905. 

"  Cements,  Limes,  and  Plasters,"  E.  C.  Eckel.     New  York,  1905. 

"  The  Mineral  Industry,"  Articles  by  Stanger  and  Blount,  and  by 
Lewis.  New  York,  1897,  1898,  1905. 

Journal  of  Society  of  Chemical  Industry.  Articles  by  B.  Blount  and 
others.  London,  1894,  1905,  1906,  and  recent  years. 

Proceedings  of  Institution  of  Civil  Engineers.  Article  by  Stanger  and 
Blount.  London,  1901. 

"  Concrete  "  (No.  2,  p.  101),  C.  H.  Desch. 

Also  H.  Le  Chatelier's  "  Experimental  Researches  on  the  Constitution 
of  Hydraulic  Mortars  "  (in  French). 

"  Cement,"  by  Bertram  Blount.     London,  1920. 


SECTION  IV.— CEEAMIG   INDUSTRIES 

Meaning  of  the  word  "  Ceramic. '  '—This  word  is  fre- 
quently stated  to  be  derived  from  the  Greek  keramos, 
"  potter's  earth  "  or  "  burned  clay  "  products.  Dr.  J.  W. 
Mellor  suggests  that  it  may  be  derived  from  the  Kerameis 
or  "  potters  "  who  lived  in  the  part  of  ancient  Athens  called 
Kerameicus,  and  who  worshipped  Keramus — the  son  of 
Bacchus.  Potters  are  called  ceramists,  and  the  potter's 
art  is  the  ceramic  art,  and  by  a  natural  tendency  the  term 
came  to  be  applied  to  clayworkers  in  general.  Some 
consider  that  it  should  refer  to  pottery  only,  and  not  to 
such  things  as  bricks,  refractory  materials,  etc.  On  the 
other  hand,  in  America  the  word  has  acquired  a  greatly 
extended  meaning.  Thus,  in  the  University  oj  Illinois 
Bulletin,  vol.  xiv.,  No.  12  (November 20, 1916), "ceramics" 
is  explicitly  stated  to  imply  "  the  technology  of  nearly  all 
mineral  products  except  ores  and  minerals  of  organic  origin. 
The  ceramic  industries  thus  embrace  the  manufacture  of 
all  kinds  of  clay  products,  such  as  stoneware,  china,  and 
porcelain  ware,  brick,  tile,  sewer-pipe,  and  terra-cotta ; 
Portland  cement,  dental  cements,  lime,  plaster,  stucco,  and 
a  variety  of  gypsum  products,  and  special  cements ;  all 
of  the  many  varieties  of  glass  and  glassware,  fused  silica, 
and  magnesia  ware  ;  enamelled  metals  and  sanitary  ware  ; 
a  variety  of  electrical  and  thermal  insulating  materials ; 
talc,  chalk,  and  slate  products ;  abrasive  materials,  such 
as  finely  divided  silica  and  carborundum  and  alundum 
products ;  rare  earth  products,  such  as  mantles  and  tips 
for  gas  burners ;  bricks,  crucibles,  and  other  refractory 

168 


CERAMIC  INDUSTRIES  169 

articles  manufactured  from  bauxite,  magnesite,  chromite, 
carbon,  graphite,  asbestos,  talc,  lime,  porcelain,  clay, 
alundum,  sand,  and  many  other  materials."  This  is  compre- 
hensive indeed  and  the  American  Ceramic  Society  apparently 
adopts  the  same  range.  The  chief  continental  technical 
journals,  such  as  Sprechsaal,  Keramische  Rundschau,  La 
Cer antique,  etc.,  do  not  accept  this  wide  interpretation  of  the 
term,  so  that  it  must  be  regarded  as  peculiar  to  America 
for  the  time  being,  though  possibly  it  might  prove  convenient 
for  adoption  at  some  future  period.  Probably  the  least 
objectionable  course  just  now  is  to  follow  Dr.  Mellor's 
view  (Trans.  Cer.  Soc.,  i69  69,  1917)  that  "  the  word  ceramic 
embraces  the  plastic  or  fictile  arts  in  which  objects  are 
first  shaped  or  moulded,  and  subsequently  baked." 

Clay  the  Basis  of  nearly  all  Ceramic  Wares. — 
From  the  earliest  period — extending  back  even  to  pre- 
historic times — the  clays  which  form  such  a  prominent 
feature  of  the  readily  accessible  surface  portions  of  the 
solid  globe  have  been  used  for  making  earthen  vessels. 
This  no  doubt  arose  naturally  from  observation  of  the 
plastic  properties  of  clay,  which  enabled  it  to  be  easily 
shaped  by  hand.  The  simple  pieces  first  made  were  for 
vast  ages  merely  dried  by  the  sun,  which  gave  rise  to  sufficient 
hardness  for  grain  and  other  dry  materials  to  be  stored  in 
them.  As  the  use  of  fire  extended  it  was  eventually  dis- 
covered that  clay  after  heating  became  much  harder  and 
stronger  than  when  simply  dried,  and  so  in  the  course  of 
time  it  became  a  settled  practice  to  bake  clay  vessels  in 
order  to  give  them  greater  hardness,  and  therefore  greater 
durability.  Sven  Oden,  a  Swedish  investigator,  defined 
clays  as  disperse  formations  of  mineral  fragments  in  which 
particles  smaller  than  2ft  (i.e.  0*002  mm.)  predominate. 
The  mineral  fragments  may  include  quartz,  felspar,  mica, 
etc.,  as  well  as  their  decomposition  products  (kaolin, 
bauxite,  hydrargillite,  zeolites,  allophane,  clays,  gelatinous 
silicic  acid,  iron  oxide,  etc.)  ;  but  none  of  these  is  necessary 
to  characterize  a  deposit  as  clay. 

The  difference  in  plasticity  of  clays,  and  the  different 


170  SILICA   AND   THE  SILICATES 

degrees  of  hardness  of  the  fired  pieces,  as  well  as  differences 
in  other  properties  such  as  colour  and  structure  after  firing, 
give  rise  to  a  great  variety  of  products.  Even  in  the  same 
district,  the  clays  would  not  all  fire  to  the  same  colour,  and 
this  fact  would  indicate  to  the  observant  a  means  of  pro- 
ducing rude  colour  decoration  by  smearing  the  surface  of 
a  piece  with  a  different  kind  of  clay  (sometimes  white),  or 
some  other  earthy  material.  The  application  of  glazes  and 
colours  later  on  modified  the  effects,  but  the  production  of 
wares  on  any  large  scale  from  purely  local  clays  continued 
with  very  few  exceptions  until  about  the  middle  of  the 
eighteenth  century.  Then  the  use  of  local  clays  was  largely 
superseded  by  the  introduction  of  white-burning  clays, 
brought  from  a  distance  in  most  cases. 

Clay  of  some  kind  continued  to  form  the  basis  of  ceramic 
wares  in  general.  Exceptions  included  such  abnormal 
products  as  those  of  the  early  Egyptians,  who  made  objects 
from  sand  mixed  with  a  little  clay,  or  carved  them  from 
solid  stone,  and  covered  them  with  bright  blue  glazes, 
fired  on  in  the  usual  way. 


GENERAL  AND  SPECIAL  CHARACTERS  OF  CLAYS  FOR 
CERAMIC  PURPOSES. 

The  most  important  characters  of  clays  for  ceramic 
purposes  are  plasticity  and  hardening  when  heated.  The 
colour  (especially  after  heating)  is  another  important 
property  in  many  cases. 

Plasticity  is  the  property  by  virtue  of  which  clay,  when 
mixed  with  a  certain  quantity  of  water,  can  be  moulded  by 
pressure  into  definite  shape.  This  property  is  lost  when 
clay  is  strongly  heated,  and  attempts  to  restore  it  after 
the  combined  water  has  been  driven  off  have  met  with  little 
success,  the  quantity  of  water  taken  up — even  on  treatment 
with  very  hot  water — being  only  small.  Quite  recently  it 
has  been  announced  by  J.  S.  Laird  and  R.  F.  Geller  (Journal 
of  American  Ceramic  Society,  2,  828,  1919)  that  although  the 
plasticity  of  raw  clays  is  little  affected  by  the  action  of  very 


CERAMIC  INDUSTRIES  171 

hot  water,  clays  which  have  been  almost  completely 
dehydrated  by  calcination  at  a  moderate  temperature 
(600°  to  700°  C.)  can  be  rehydrated  by  heating  with  water 
to  200°  to  270°  C.  (under  great  pressure  of  course)  for  8  to 
48  hours.  The  rehydrated  material  is  generally  plastic 
and  seems  to  be  colloidal.  After  being  worked  up  and 
dried  two  or  three  times  it  resembles  a  raw  clay,  that  is, 
its  original  condition.  Clays  which  have  been  calcined  at 
higher  temperatures  can  be  rehydrated  only  much  more 
slowly  and  incompletely.  The  greater  the  plasticity  of 
clay  the  more  water  is  generally  required  to  bring  it  to  a 
given  degree  of  softness  for  working. 

Plasticity  is  not  a  simple  measurable  property,  but  accord- 
ing to  L,e  Chatelier  results  from  the  union  of  two  elementary 
properties  which  can  be  measured  separately  ;  these  are 
deformability  (or  deformation  before  rupture),  and  tenacity 
(or  resistance  of  the  moist  body  to  deformation).  The 
body  for  ceramic  pressed  tiles  may  have  very  low  deforma- 
bility, and  so  may  be  very  silicious  or  sandy.  Delicate 
vases,  if  they  are  to  be  turned,  have  on  the  contrary  to  be 
made  from  a  very  deformable  body.  Resistance  of  the 
moist  body  is  very  important  for  large  vases,  but  is  of  little 
consequence  for  flat  articles  fired  horizontally.  Each  of  the 
two  factors  of  plasticity  can  be  measured  by  the  same 
methods  as  are  used  for  metals,  plasticity  being  quite 
comparable  to  malleability,  though  the  magnitude  is  usually 
far  greater  in  metals  than  in  ceramic  bodies.  Tenacity  of 
ceramic  bodies  may  be  expressed  in  kilogs.  per  sq.  cm.  and 
deformability  as  linear  expansion  (per  cent,  of  original 
length).  Both  resistance  to  deformation  and  magnitude 
of  deformation  increase  with  speed  of  operation  within  very 
wide  limits,  and  this  serves  to  explain  the  differences  often 
observed  in  results  obtained  from  different  workmen  who 
may  at  first  sight  seem  to  be  working  in  exactly  the  same 
way  ;  probably  the  best  results  come  from  the  workman 
who  brings  the  body  to  its  proper  shape  in  the  shortest 
possible  time. 

Plasticity  depends  on  various  factors,  the  more  important 


172  SILICA   AND   THE  SILICATES 

being  the  proportion  of  mixing  water,  the  size  and  shape  of 
the  solid  particles,  and  the  nature  of  the  material  dissolved 
in  the  water.  Either  too  little  water  or  too  much  gives  no 
plasticity,  owing  to  absence  of  deformability  or  absence  of 
resistance  respectively.  A  normal  body  mixture  is  taken 
as  being  the  richest  in  water  which  can  be  worked  without 
sticking  too  much  to  the  hand  ;  though  the  definition  may 
seem  rather  vague  it  has  been  found  (still  relying  on  the 
authority  of  I^e  Chatelier)  that  different  operators  agree  to 
within  I  per  cent,  on  the  water  content  of  this  normal  body. 
Some  clays  give  a  normal  body  with  30  per  cent,  water. 
Coarser  clay  needs  less  water  to  give  a  normal  body,  and 
this  applies  also  when  sand,  chalk,  etc.,  are  mixed  with  clay. 
Dissolved  substances  may  greatly  affect  the  plasticity  of 
clay  bodies.  Alkalies  (potash,  soda,  etc.),  as  also  their 
carbonates  or  silicates,  considerably  reduce  the  amount 
of  water  necessary  as  compared  with  pure  water,  but 
alkaline  earths  (magnesia,  lime,  strontia,  baryta) — and 
to  a  smaller  extent  their  carbonates,  sulphates,  etc.  (when 
soluble) — increase  the  amount  of  water  necessary.  Tannin 
and  certain  other  organic  substances  act  like  the  alkalies. 
Reduction  of  the  mixing  water  reduces  also  the  contraction 
in  drying. 

The  plasticity  of  clay  is  due  apparently  to  the  extreme 
fineness  of  the  particles  and  their  lamellar  structure.  In 
fact,  all  very  fine  powders,  whatever  their  chemical  nature 
may  be,  possess  some  degree  of  plasticity,  though  much 
inferior  to  that  of  clay  ;  they  can  be  kneaded  in  the  fingers, 
and  acquire  some  hardness  by  drying. 

The  combination  of  lamellar  structure  with  extreme 
fineness  is  very  rare  outside  clays,  but  I^e  Chatelier  has 
obtained  it  by  grinding  of  mica  and  of  glauconite,  neither 
of  which  occurs  naturally  in  fine  powder.  The  material 
thus  obtained  was  as  plastic  as  clay,  though  prolonged 
grinding  was  necessary  in  the  case  of  mica,  owing  to  the 
elasticity  of  the  lamellae.  Glauconite  occurs  naturally  as  a 
coarse  sand  consisting  of  nearly  spherical  grains.  Ground 
glauconite  can  be  treated  like  clay  for  making  into  a  normal 


CERAMIC  INDUSTRIES  173 

body  or  a  slip.  Plasticity  is  a  direct  consequence  of  the 
existence  of  a  superficial  tension  in  the  liquid,  the  resultant 
of  the  capillary  tensions  in  the  many  minute  air  spaces 
always  present  in  a  plastic  body.  As  evaporation  proceeds, 
water  is  concentrated  more  and  more  closely  in  the  spaces, 
and  the  clay  lamellae  become  more  strongly  pressed  against 
one  another  and  so  adhere  together,  the  average  force  of 
adhesion  increasing  with  the  number  of  grains  on  unit 
surface,  that  is,  with  increased  fineness  of  the  clay  particles. 
The  degree  of  hardness  when  dried  usually  increases  with 
the  plasticity  of  the  clay. 

Some  authorities  ascribe  the  plasticity  of  moist  clay  to 
the  presence  of  colloidal  grains  (or  glue-like  particles) 
composed  of  hydrated  metallic  oxides  or  salt  in  a  peculiar 
non-crystalline  condition.  The  water  content  of  these 
particles  varies  continually  with  the  temperature  and 
with  the  vapour  pressure  of  the  surrounding  atmosphere. 
The  components  of  clay  which  might  take  a  colloidal  form 
include  aluminium  hydroxide,  iron  oxide,  and  hydrated 
silicic  acid.  Still  other  authorities  refer  plasticity  to  mole- 
cular attraction  between  the  clay  particles  themselves,  or 
between  them  and  the  water  surrounding  them.  It  scarcely 
seems  likely  that  any  of  these  suggested  causes  of  plasticity 
is  the  sole  cause,  but  that  plasticity  is  rather  dependent  on 
a  combination  of  them. 

Contraction  or  Shrinkage. — During  the  evaporation 
of  the  water  the  grains  or  particles  of  solid  material  approach 
one  another  owing  to  capillary  tensions,  the  effects  of  which 
increase  as  they  come  to  act  in  narrower  and  narrower 
spaces.  With  very  fine  clays  rich  in  kaolinite  this  contrac- 
tion in  drying  may  amount  to  20  per  cent,  of  the  linear 
dimensions. 

The  further  contraction  during  firing  is  never  uniform, 
but  is  produced  first  towards  the  outside,  and  the  resistance 
offered  by  the  uncontr acted  central  parts  results  in  the 
formation  of  surface  cracks,  which  gradually  extend  inwards 
and  cause  the  piece  to  be  broken  into  several  parts.  To 
remedy  this  trouble,  the  clay  is  mixed  with  non-contracting 


174  SILICA   AND   THE  SILICATES 

materials  such  as  grains  of  sand  or  of  grog  (calcined  clay). 
The  presence  of  these  materials  does  not  prevent  the 
contraction  and  cracking  of  the  clay,  but  the  distribution 
of  the  cracks  is  altered,  so  that  they  are  rendered  harmless. 
The  less  the  proportion  of  clay  used  in  the  bodies  the  less 
the  contraction,  but  also  the  less  the  final  strength.  In 
industrial  clay  bodies  the  proportion  of  true  clay  ranges 
from  about  25  per  cent,  in  bricks  and  other  common  articles 
to  50  per  cent,  or  a  little  more  in  glass-house  pots. 

The  contraction  of  a  clay  body  helps  to  loosen  it  from  the 
moulds. 

Contraction  is  nearly  finished  when  only  half  of  the 
mixing  water  has  evaporated,  so  the  later  stages  of  the 
drying  may  be  carried  out  very  rapidly  without  risk  of 
producing  cracks.  In  the  case  of  bricks  and  other  thick 
ware,  the  drying  is  commenced  behind  cloths  so  as  to  retard 
the  drying,  which  is  finished  in  full  sunshine  (or  other 
heat). 

The  composition  can  be  so  adjusted  that  by  using 
clays  containing  sufficient  sand,  and  suitably  regulating 
the  firing  temperature,  bricks  can  be  made  which  do  not 
change  their  dimensions  as  the  result  of  firing. 

Porosity  of  a  clay  arises  from  the  partial  replacement 
of  the  water  (after  evaporation)  by  air  ;  the  contraction 
which  accompanies  the  evaporation  prevents  complete 
replacement.  Numerically  the  porosity  is  the  total  volume 
of  the  pore  spaces  expressed  as  a  percentage  of  the  apparent 
volume  of  the  dry  or  fired  material,  depending  on  the  shape 
and  size  of  the  particles  ;  pore  space  generally  increases  as 
fineness  of  particles  increases.  When  clay  consists  of 
particles  of  different  sizes,  the  porosity  is  much  lessened. 

The  possible  rate  of  safe  drying  depends  on  the  quantity 
of  water  absorbed  and  the  facility  with  which  it  can  escape  ; 
large  pores  permit  rapid  escape,  but  small  pores  retard 
both  absorption  and  evaporation  of  the  water. 

At  the  moment  of  dehydration  the  volume  of  the  pores 
increases  suddenly  about  10  per  cent,  by  liberation  of  com- 
bined water.  As  the  temperature  is  carried  higher,  the 


CERAMIC  INDUSTRIES  175 

porosity  decreases  as  contraction  increases.  The  use  of 
non-plastic  materials  to  mix  with  clay,  by  reducing  contrac- 
tion, tends  to  increase  porosity. 

Expansion  of  clay  bodies  causes  great  difficulty  in  the 
application  of  vitreous  glazes.  Thus  fine  earthenware  often 
develops  very  numerous  fine  cracks  in  the  glaze.  The  expan- 
sion of  glasses  and  glazes  is  generally  greater  than  that  of  clay 
bodies,  so  on  cooling  the  glaze  remains  in  a  state  of  tension, 
and  finally  proceeds  to  crack.  This  cracking  of  the  glaze — 
termed  crazing — takes  place  rapidly  on  exposure  to  con- 
siderable variations  of  temperature,  as  when  domestic  ware 
is  washed  in  hot  water.  In  exceptional  wares  the  body 
may  contract  more  than  the  glaze,  and  its  cracking  involves 
the  destruction  of  the  glaze  ;  sometimes  the  body  resists 
cracking,  and  the  compressed  glaze  peels  off. 

Tensile  Strength,  or  resistance  of  clay  to  rupture  when 
air-dried,  has  importance  in  connection  with  the  handling, 
moulding,  and  drying  of  ware.  The  tensile  strength  of  clay 
ranges  from  a  few  pounds  to  over  400  pounds  per  square  inch. 

Fusibility. — The  temperature  of  fusion  or  complete 
softening  of  clay  depends  on  the  amount  of  fluxes  present 
and  their  nature,  the  size  of  the  grains  (especially  of  refrac- 
tory particles),  the  homogeneity  of  the  mass,  the  kind  of 
atmosphere  (oxidizing  or  reducing),  and  the  mode  of  com- 
bination of  the  elements  in  the  clay.  Usually  no  reaction 
takes  place  between  the  grains  until  one  kind  melts,  but  it 
is  not  necessary  for  each  to  melt  in  order  that  reaction  may 
take  place. 

The  white-burning  clays  are  characterized  by  the  low 
proportion  of  iron  oxide  present,  and  the  high  percentage 
of  alumina.  In  the  China  clays  (kaolins)  the  alkalies  are 
very  low,  and  so  are  all  oxides  other  than  silica,  alumina, 
and  water,  so  that  China  clays  are  as  a  class  highly  refractory. 
Ball  clays  contain  a  greater  percentage  of  alkalies,  and  of 
silica,  which,  in  conjunction  with  the  exceptionally  fine  state 
of  subdivision  of  the  particles,  makes  them  distinctly  less 
refractory  than  China  clays.  Ball  clays  are  also  characterized 
by  the  presence  of  fine  particles  of  carbonaceous  material, 


176  SILICA   AND   THE  SILICATES 

which  gives  them  a  dark  blue  or  almost  black  appearance 
in  their  natural  condition. 

The  best  fireclays  have  strong  points  of  resemblance 
with  the  white  refractory  clays,  the  chief  difference  being 
in  the  larger  percentage  of  iron  oxide  present.  This  similarity 
is  enhanced  as  the  result  of  electro-osmosis  treatment  of  such 
fireclays.  The  author  had  occasion  a  few  years  since,  to 
prepare  some  small  trials  in  which  a  South  Staffordshire 
fireclay  so  treated  was  substituted  for  the  ball  clay  con- 
stituent of  an  earthenware  body.  The  only  distinguishable 
feature  in  the  resulting  trials  after  the  glost  firing,  was  that 
they  were  slightly  more  yellow  than  normal  pieces  containing 
ball  clay. 

CORNISH  STONE  AND  FELSPAR. 

These  are  two  important  fluxing  materials  largely 
used  in  the  ceramic  industry.  They  are  used  in  finely 
divided  condition,  chiefly  in  bodies  and  glazes,  and  the 
object  with  each  is  the  same,  that  is,  to  assist  in  promoting 
more  or  less  vitrification  of  the  mixture  under  suitable 
heat  treatment.  But  whilst  complete  fusion  of  the  whole 
is  aimed  at  in  glazes,  the  intention  in  bodies  is  generally 
to  produce  a  sufficient  degree  of  vitrification  to  give  the 
fired  material  a  varying  amount  of  hardness  and  strength 
according  to  the  nature  of  the  particular  product  desired. 
In  the  cases  of  stoneware  and  porcelain,  the  vitrification 
is  carried  almost  to  completion,  and  with  porcelain  especially 
great  care  has  to  be  exercised  to  avoid  distortion  of  the  ware 
when  softened  by  heat. 

Cornish  Stone  or  China  Stone  is  largely  worked  in 
Cornwall  and  Devonshire.  It  was  discovered  by  William 
Cookworthy,  a  Plymouth  apothecary,  in  Cornwall,  between 
1745  and  1750,  and  was  first  used  by  him  for  making  porce- 
lain. A  common  type  of  granite  consists  essentially  of 
the  minerals  quartz,  felspar  (orthoclase),  and  white  mica 
(muscovite),  with  often  small  amounts  of  other  minerals 
such  as  tourmaline,  fluor  spar,  topaz,  etc.  Decomposition 


CERAMIC  INDUSTRIES  177 

of  the  felspar  of  these  granites  gives  rise  to  formation  of 
China  clay,  and  Cornish  stone  is  a  similar  rock  in  an  inter- 
mediate state,  that  is,  with  the  felspar  more  or  less  decom- 
posed. Cornish  stone  thus  consists  essentially  of  quartz, 
white  mica,  felspar  in  a  more  or  less  advanced  state  of 
decomposition,  with  possibly  other  minerals  in  small 
proportions.  Felspar  is  light-coloured  (whitish  or  creamy), 
quartz  appears  dark-greyish  in  the  rock,  though  the  indi- 
vidual grains  are  glassy  and  nearly  colourless,  mica  is  whitish, 
but  sometimes  dark.  The  different  commercial  varieties 
depend  on  the  extent  of  the  decomposition  of  the  felspar, 
and  the  nature  and  amount  of  the  accessory  minerals. 
When  the  felspar  is  only  in  an  early  stage  of  decomposition, 
the  stone  is  said  to  be  hard — purple  or  white  as  the  case 
may  be.  In  medium  white  stone  the  decomposition  may 
be  sufficient  to  make  the  material  friable,  though  the  stone 
is  fairly  compact.  In  hard  white  stone  decomposition  of 
the  felspar  has  just  begun,  and  in  hard  purple  there  is  little 
or  no  appearance  of  decomposition. 

Cornish  stone,  as  well  as  China  clay,  was  first  used  by 
William  Cookworthy  (of  Plymouth),  who  took  out  a  patent 
(No.  898,  March  17,  1768).  This  was  taken  over  by  Richard 
Champion  of  Bristol,  who  obtained  an  extension  of  the 
patent  (No.  1096,  September  15,1775).  In  1881  Champion 
sold  his  patent  to  seven  North  Staffordshire  potters. 

Jersey  stone  resembles  Cornish  stone,  but  is  somewhat 
more  fusible  and  contains  rather  more  iron.  It  has  been 
suggested  that  it  would  be  very  suitable  for  use  in  glazes. 

Felspar,  as  used  in  this  country,  is  mostly  from  Norway 
and  Sweden.  It  is  the  potash  felspar  (orthoclase)  and 
usually  is  salmon-coloured  or  pale  reddish-brown.  Felspar 
is  hard  and  compact,  but  when  finely  ground  it  fuses  rather 
easily  to  a  colourless  or  whitish  mass.  On  the  other  hand, 
some  white  felspars  become  grey  after  melting.  At  high 
temperatures  felspar  forms  in  thin  layers  a  perfectly  trans- 
parent glass  or  glaze. 

For  ceramic  uses,  felspar  is  crushed  and  afterwards 
ground. 

c.  12 


178  SILICA   AND  THE  SILICATES 

VARIETIES  OF  POTTERY  AND  PORCELAIN. 

It  is  impossible  to  assign  sharp  limits  for  the  different 
kinds  of  ware,  because  the  same  materials  may  give  different 
kinds  under  different  conditions,  but  they  may  be  con- 
veniently (though  rather  broadly)  grouped  into  three 
divisions. 

I.  Bodies  made  from  only  plastic  materials  (one  or  more 
clays),   or  such  materials  mixed  naturally   or   artificially 
with   a   proper   proportion   of   non-plastic   materials    (like 
flint  or  other  form  of  silica,  or  ground  pitchers,  etc.),  but 
without  the  addition  of  any  fluxing  materials.     The  firing 
is  not  carried  to  the  fusing  point,  and  the  products  are 
highly  porous,  opaque,  fairly  hard,  and  usually  of  a  red- 
brown,  yellow,  or  sometimes  grey  to  black  colour.     In  this 
group   are  included  common  bricks,   flooring  and  roofing 
tiles,  terra-cotta   (mostly  red  or    brown),   refractory  clay 
products,  and  common  stoneware.     Terra-cotta  is  made  of 
common  clay,  always  containing  iron,  and  is  not  adapted 
for  withstanding  great  heat ;    some    special    terra -cottas 
are  white  or  nearly  so.     Refractory  clay  products  are  made 
from    clays   containing    comparatively    small    amounts    of 
impurities,   free  especially   from  iron,   lime,    and   alkalies, 
and  which  can  therefore  resist  high  temperatures  ;    they 
include  firebricks  and  special  shapes  for  use  in  connection 
with  heating  apparatus  (as  furnace  linings,  etc.). 

Terra-cotta  is  sometimes  covered  with  a  transparent 
lead  glaze  or  an  opaque  lead  and  tin  glaze,  which  makes  the 
surface  impermeable  to  liquids,  and  often  also  masks  the 
colour  of  the  body  ;  the  tin  glaze  is  usually  white,  but  the 
transparent  lead  glaze  is  often  dark-coloured. 

II.  Bodies  containing  plastic,   non-plastic,   and  fluxing 
materials  (which  help  to  make  the  mixture  fusible)  ;    the 
bodies  after  firing  are  white  or  light-coloured,   compact, 
hard,  and  usually  opaque.     They  include  fine  earthenware 
(or    faience),    fine    stoneware,    and    mortar    bodies.    Fine 
earthenware  has  an  opaque  white  body  fired  to  incipient 
fusion,  and  covered  with  a  transparent  colourless  (alkali-lead 


CERAMIC  INDUSTRIES  179 

or  sometimes  alkali-boracic  leadless)  glaze  adapted  for 
underglaze  decoration.  Stoneware  has  an  opaque  body 
including  clay  mixed  with  a  suitable  proportion  of  flux, 
and  heated  to  the  point  of  complete  softening,  so  that  it 
becomes  hard  and  very  impermeable.  Mortar  bodies  are 
only  specially  hard  white  stonewares. 

III.  Bodies  containing  only  plastic  and  fluxing  materials, 
the  fired  bodies  being  translucent,  white  or  cream-coloured, 
hard,  and  durable.  They  include  porcelains  (hard  and 
soft,  the  latter  both  natural  and  artificial),  parian,  jasper, 
etc.  Sometimes  the  body  is  coloured  by  material  specially 
added. 

Ceramic  bodies  are  often  divided  primarily  into  permeable 
and  impermeable  bodies.  The  permeable  bodies  include 
those  in  Class  I.  with  the  exception  of  common  stoneware, 
and  those  in  Class  II.  excepting  fine  stoneware  and  mortar 
bodies  ;  impermeable  bodies  include  these  exceptions  along 
with  the  bodies  grouped  in  Class  III.  Impermeable  pottery 
has  a  vitreous  fracture,  is  smooth  to  the  tongue,  and  cannot 
be  scratched  by  steel.  Permeable  pottery  has  an  earthy 
fracture,  is  rough  to  the  tongue,  is  generally  soft,  and  can 
in  nearly  all  cases  be  scratched  by  steel.  This  distinction 
into  permeable  and  impermeable  bodies  is  largely  a  matter 
of  firing,  as  permeable  bodies  become  impermeable  if  heated 
strongly  enough  (though  the  ware  may  then  lose  its  shape), 
and  impermeable  bodies  are  always  permeable  during  the 
earlier  stages  of  the  firing. 


PROCESSES  IN  THE  PREPARATION  OF  EARTHENWARE. 
GRINDING  AND  MIXING. 

Clays  do  not  require  grinding.  China  clays  are  purified 
at  the  pit  or  mine,  by  washing  and  successive  sedimentations, 
the  impurities  being  mostly  deposited  in  the  earlier  stages, 
and  nearly  pure  clay  in  the  later  stages.  Ball  clay  is  not 
subjected  to  any  special  treatment  at  the  pit,  but  should 
always  be  weathered  before  being  used ;  for  this  purpose 
it  is  most  advantageous  to  spread  it  out  for  exposure  with 


i8o  SILICA   AND   THE  SILICATES 

the  long  slope  facing  the  south  (so  as  to  get  maximum  solar 
radiation),  the  superficial  portions  being  raked  off  from 
time  to  time  after  it  has  weathered  sufficiently. 

Flint  is  the  non-plastic  material  mostly  used  in  England 
for  mixing  with  clay  and  fluxing  material  to  make  earthen- 
ware ;  in  other  countries  ground  sand  or  ground  quartz 
is  often  used  instead.  The  flint  used  is  generally  obtained 
from  chalk  flints,  which  are  very  hard  and  solid.  Before 
grinding,  flints  are  calcined  to  serve  two  useful  purposes  ; 
they  become  much  more  friable  (and  so  are  more  easily 
crushed),  and  the  flint  substance  undergoes  a  certain  bene- 
ficial change  in  its  structure,  rendering  it  better  adapted  for 
use  in  ceramic  bodies.  If  calcined  at  about  800°  to  900°  C., 
the  quartzose  flint  changes  to  tiidymite,  the  specific  gravity 
being  reduced  at  the  same  time  irom  about  2*6  to  possibly 
about  2 '33  (or  a  little  higher)  with  a  corresponding  change 
in  volume.  If  the  heating  be  carried  to  about  1300°  C.  or 
higher,  the  tridymite  in  its  turn  changes  more  or  less  com- 
pletely into  cristobalite,  with  a  further  small  reduction  in 
specific  gravity  and  increase  in  volume.  If  such  changes 
took  place  during  the  firing  of  ware  containing  flint  they 
would  be  likely  to  cause  serious  trouble. 

Flint  makes  the  fired  earthenware  body  more  open  and 
porous,  reduces  the  excessive  plasticity  imparted  by  ball 
clay,  and  so  promotes  quicker  drying.  It  makes  the  body 
whiter  and  stronger,  and  enables  it  to  resist  a  high  tem- 
perature without  becoming  deformed.  An  addition  of 
flint  to  an  earthenware  body  is  a  good  remedy  for  crazing 
(that  is,  formation  of  fine  cracks  in  the  glaze),  but  an  excess 
of  flint  is  liable  to  cause  peeling  (scaling  of  the  glaze  from  the 
body). 

It  is  interesting  to  note  that  from  the  commencement 
of  its  use  for  pottery  in  Staffordshire,  flint  was  pounded  or 
beaten  dry,  followed  by  sifting,  until  1726,  when  Thomas 
Benson  took  out  a  patent  (No.  487,  dated  November  5)  for 
wet  grinding  by  means  of  iron  edge-runners  worked  by  a 
water-wheel ;  the  grinding  was  finished  on  iron  pans  with 
large  iron  balls.  In  1732,  Benson  obtained  a  patent  (No.  536, 


CERAMIC  INDUSTRIES 


181 


January  14)  for  an  improved  process,  the  improvements  con- 
sisting in  the  substitution  of  stone  edge-runners  and  pans, 
with  large  stone  balls,  to  take  the  place  of  iron  ;  wind  and 
horses  are  mentioned  as  additional  sources  of  power  to  work 
the  wheels. 

It  may  be  noted  that  calcined  and  ground  flint  was 
used  as  early  as  1689  by  D wight  of  Fulham,  but  the  flint 
proved  more  expensive  and  troublesome  than  the  white 
sand  which  it  replaced.  It  was  about  fifty  years  later  that 
it  came  into  use  to  improve  white  stoneware  in  Staffordshire. 


FIG.  ii. — Pulverizing  mill  with  solid  bottom. 
(Manor  Engineering  Co.,  Fenton.) 

Wedgwood  (in  a  letter  to  Bentley,  dated  July  19,  1777) 
attributed  the  discovery  to  a  Shelton  potter  named  Heath, 
but  Simeon  Shaw  and  Parkes,  writing  much  later,  credit 
Thomas  Astbury  (the  younger)  with  the  invention. 

The  calcined  flints  are  graded,  the  finer  material  being 
removed  and  the  remainder  crushed  ready  for  grinding  on 
a  pan  or  in  a  cylinder.  The  crushing  is  usually  done  in  a 
jaw  crusher. 

The  grinding  pan  is  usually  circular  in  section,  but 
sloping  a  little  outwards  from  below  upwards.  The  bottom 


182  SILICA   AND  THE  SILICATES 

of  the  pan  is  formed  by  a  thick  pavement  consisting  of 
blocks  of  hard  chert,  and  over  the  top  of  this  pavement 
larger  blocks  of  chert  (called  runners)  are  carried  by  arms 
fixed  on  a  revolving  central  shaft.  The  crushed  material 
is  charged  on  to  the  pavement,  some  water  is  added,  and 
the  material  is  gradually  reduced  to  a  fine  state  of  sub- 
division, the  time  required  depending  on  the  weight  of  the 
runners.  Excess  of  water  tends  to  prolong  the  time  of 
grinding.  This  form  of  pan  was  once  exclusively  used  for 
the  fine  grinding  of  all  pottery  materials.  For  the  pave- 
ment is  used  Welsh  chert  from  Flintshire,  etc.,  or  Derbyshire 
chert  from  near  Bakewell,  or  occasionally  Scotch  granite 
from  Skye  ;  the  runners  come  mostly  from  near  Bakewell. 
The  chert  is  associated  with  limestone,  and  the  blocks  used 
should  be  as  free  as  possible  from  lime  compounds.  The 
flint  is  usually  fine  enough  after  n  or  12  hours'  grinding 
on  the  pan,  but  still  contains  some  comparatively  coarse 
material.  It  is  then  run  into  washing  tanks,  where  the 
coarser  particles  settle,  and  the  finer  material  is  gradually 
run  off  by  removing  plugs  at  progressively  lower  levels. 

The  grinding  cylinder  is  generally  an  iron  drum  lined 
with  some  hard  material  such  as  porcelain  or  quartz,  etc. 
Besides  the  material  to  be  ground,  the  cylinder  also  contains 
flint  pebbles  or  hard  porcelain  balls.  When  a  hard  porce- 
lain lining  is  used,  its  surface  should  be  rough  to  give  good 
results,  and  this  may  be  ensured  by  the  introduction  of 
sharp  pieces  of  flint.  As  the  cylinder  revolves  (on  a  hori- 
zontal axis),  the  pebbles  or  balls  exert  a  crushing  and 
grinding  action  on  the  material,  which  is  caught  between 
the  pebbles  and  the  sides,  and  between  the  pebbles  them- 
selves. The  size  and  number  of  the  pebbles  need  adjusting, 
and  also  the  number  of  revolutions  per  minute. 

An  important  difference  between  the  pan  and  the  cylinder 
is  that  in  the  latter  either  wet  or  dry  material  can  be  ground. 
It  is  claimed  that  a  cylinder  can  grind  much  more  material 
than  a  pan  in  a  given  time,  and  with  much  less  expenditure 
of  power.  On  the  other  hand,  it  is  objected  that  cylinder- 
ground  material  varies  less  than  pan-ground  material  as 


CERAMIC   INDUSTRIES  183 

regards  sizes  of  particles,  that  in  particular  it  contains 
relatively  much  less  of  the  very  fine  particles,  so  that  the 
packing  is  not  so  close,  that  the  particles  are  less  irregular 
than  when  pan-ground,  that  ware  made  from  it  is  more 
liable  to  dunt  (or  develop  fine  cracks  in  the  body),  that 
flat  ware  made  from  it  is  less  able  to  support  its  own  weight 
when  arranged  in  bungs  in  the  warehouse,  and  that  generally 
ware  made  from  it  gives  more  pitchers  (or  broken  articles). 
There  is,  however,  evidence  to  show  that  with  due  care 
and  proper  regulation  of  the  grinding  process,  cylinder- 
ground  materials  can  be  produced  fine  enough  to  give  as 
satisfactory  results  as  pan-ground  materials — at  any  rate 
for  earthenware,  though  for  china  it  does  not  seem  to  have 
proved  successful  hitherto,  possibly  because  of  the  lower 
degree  of  plasticity  and  tenacity  of  unfired  china  bodies. 
Part,  at  least,  of  the  difference  between  pan-ground  material 
and  cylinder-ground  material  is  due  to  the  fact  that  the 
former  is  washed,  but  the  latter  is  nearly  always  sieved 
without  washing,  as  otherwise  the  output  would  be  very 
much  reduced. 

Stone  is  not  calcined,  but  after  crushing  is  put  on  the 
pan,  and  under  proper  conditions  is  well  ground  in  n  or  12 
hours.  As  in  the  case  of  flint,  the  ground  stone  needs 
washing  to  take  out  the  coarser  particles. 

Felspar  is  treated  much  like  stone,  but  usually  takes 
longer  to  grind,  and  when  ground  it  should  be  run  direct 
on  to  the  drying  kiln,  as  it  cannot  easily  be  kept  in  a  slop 
state  owing  to  its  strong  tendency  to  set. 

Bone  is  calcined  before  grinding.  The  grinding  usually 
takes  about  10  hours,  and  the  ground  product  has  to  be 
carefully  washed  like  ground  flint  and  stone. 

Clays  are  prepared  by  simply  blunging  or  slipping,  that 
is,  agitating  with  water  in  a  sort  of  tank  called  a  blunger, 
which  is  octagonal  or  hexagonal  in  cross  section,  having 
iron  sides,  with  usually  a  paved  bottom,  and  a  wooden 
cover.  The  central  vertical  shaft  carrying  obliquely  set 
flat  arms  is  caused  to  rotate,  the  arms  serving  to  lift  the 
material  to  the  top  of  the  blunger,  where  it  is  received  by 


184 


SILICA   AND   THE  SILICATES 


iron  splash  boards  fastened  to  the  sides,  and  the  slip  falls 
to  the  bottom,  the  slip  thus  becoming  thoroughly  mixed. 

The  clay  in  the  blunger  is  mixed  with  suitable  amounts 
of  water — adjusted  when  necessary  by  adding  moie  clay 
or  more  water — to  give  a  slip  of  definite  standard  weight, 
which  is  usually  24  oz.  to  the  pint  in  the  case  of  ball  clay, 
and  26  oz.  to  the  pint  for  China  clay.  Water  alone  weighs 
20  oz.  to  the  pint.  The  writer  once  had  the  singular  experi- 
ence of  meeting  with  ball  clay  "  slip  "  which  weighed  less 
than  20  oz.  to  the  pint — actually  less  than  the  same  bulk 


FIG.  12.— Over-geared  slip  blunger.     (Manor  Engineering  Co.,  Fenton.) 

of  water.  It  had  another  peculiarity  in  that  it  would  not 
flow,  even  when  a  containing  vessel  was  turned  upside 
down.  Continued  blunging  caused  no  improvement,  and 
the  sliphouse  operations  became  somewhat  disorganized. 
Close  examination  of  the  ball  clay  slip  under  a  lens  revealed 
the  presence  of  innumerable  minute  bubbles  of  gas,  probably 
air  entangled  in  the  slip  owing  to  the  presence  of  some  oily 
liquid  which  had  got  into  the  water.  The  most  obvious 
remedy  was  the  application  of  heat  in  order  to  expand 
the  bubbles,  and  this  was  successfully  accomplished  by 


CERAMIC  INDUSTRIES  185 

temporarily  fixing  a  branch  steam-pipe  with  its  open  end 
near  the  bottom  of  the  blunger. 

The  dry  material  in  a  standard  pint  of  ball  clay  slip 
averages  about  6J  oz.,  and  China  clay  slip  contains  9  to  10  oz. 
of  dry  solid  in  the  standard  pint. 

Dry  Mixing.— The  mixing  of  dry  materials  by  measuring 
(as  in  boxes)  is  not  practised  for  any  of  the  finer  wares,  and 
the  method  could  not  give  very  consistent  results  unless 
the  materials  were  uniform  as  regards  size  of  grain.  Careful 
weighing  of  finely  divided  dry  materials  is  commonly  resorted 
to  for  the  finest  bodies  (allowance  being  made  for  moisture, 
etc.),  and  the  mixture  is  then  blunged  or  slipped,  that  is, 
well  agitated  with  water  to  form  a  slip  or  slop.  The  dry 
method  of  mixing  requires  less  power  and  less  space  ;  it 
needs  no  mixing  ark,  and  only  one  blunger.  The  wet  method 
requires  more  power  and  machinery,  and  more  space.  The 
dry  method  is  more  expensive  as  regards  labour  and  raw 
materials,  for  the  flint  and  stone  have  to  be  dried  after 
wet  grinding. 

Wet  Mixing.— In  this  method,  which  is  most  commonly 
employed,  the  materials  (finely  divided)  are  first  blunged 
separately,  and  the  resulting  slips  (of  definite  pint  weights) 
are  run  separately  into  a  mixing  ark  or  large  vat,  where 
they  are  well  mixed  together  by  agitation  with  revolving 
arms.  The  usual  means  of  getting  the  right  proportions 
is  to  fill  up  to  successive  marks  on  an  upright  stick  with 
the  several  slips — in  the  case  of  earthenware,  ball  clay, 
China  clay,  flint,  and  Cornish  stone,  in  the  order  named. 
Sometimes  the  stick  is  marked  in  inches  (instead  of  the 
four  special  marks),  and  sometimes  the  side  of  the  mixing 
ark  is  similarly  marked,  a  specified  height  being  filled  up 
in  either  case  with  each  of  the  several  slips.  A  small  but 
definite  amount  of  stain — also  in  slop  state — is  added  to 
each  mixing  to  correct  the  slight  creamy  or  yellowish  tint 
which  the  ware  would  otherwise  have  after  firing.  The 
blue  stain  consists  of  cobalt  oxide  mixed  with  flint,  stone, 
and  other  white  substances,  calcined,  and  then  finely  ground. 
Sometimes  solution  of  cobalt  chloride  or  sulphate  is  used 


i86  SILICA   AND   THE  SILICATES 

instead  for  staining.  A  certain  amount  of  blunged  "  cuttings  " 
(or  scraps  of  clay  from  broken  unfired  ware,  odds  and  ends 
of  plastic  clay,  etc.)  is  added  to  each  mixing  of  slip.  Such 
material  by  itself  would  give  a  more  plastic  clay  than  that 
freshly  prepared,  and  so  increases  the  plasticity  of  the  full 
mixing. 

This  usual  method  of  blending  or  mixing  slips  or  slop 
materials  cannot  be  regarded  as  very  reliable,  because  it 
is  practically  impossible  to  gauge  accurately  the  materials 
passing  into  the  large  mixing  arks,  and  it  is  also  difficult  to 
make  proper  allowance  for  slight  deviations  in  pint  weights 
or  densities  of  the  prepared  slop  materials.  Ordinary 
earthenware  body  slip  is  about  26  oz.  to  the  pint,  including 
about  9j  oz.  of  dry  material. 

It  should  have  been  mentioned  that  in  running  the 
different  slops  into  the  mixing  ark,  coarse  sieves  are  generally 
used  to  catch  any  lumps  of  clay  or  dirt,  etc.  A  fine  sieve  is 
used  for  the  stain  to  ensure  that  none  but  very  fine  particles 
pass  into  the  mixture. 

The  slip  when  properly  mixed  is  passed  through  a  series 
of  lawns  or  sieves  to  intercept  any  coarse  particles  which 
may  accidentally  get  in  the  slip.  Such  particles  may 
include  iron  pyrites,  lignite,  quartz,  sand,  small  lumps  of 
clay,  coarse  flint  or  stone  particles,  and  occasionally  other 
foreign  material.  Very  coarse  material  generally  settles 
at  the  bottom  of  the  blunger,  which  should  be  cleaned  out 
from  time  to  time.  Iron  pyrites  if  not  removed  would  cause 
specks  in  the  ware,  or  even  blisters.  Lignite  would  either 
burn  away  and  leave  surface  holes  or  black  specks,  according 
as  the  supply  of  oxygen  were  adequate  or  not.  Sand  grains 
would  also  be  liable  to  cause  specks,  as  sand  grains  often 
have  thin  coatings  of  iron  oxide. 

The  lawns  for  sifting  are  made  of  silk,  brass,  or  phosphor 
bronze,  and  should  be  kept  clean  so  that  the  slip  shall  not 
splash  over  the  sides.  There  are  three  main  types  of  lawn 
or  sieve  used  in  the  pottery  industry,  the  three-decked 
lawn,  the  cradle  lawn,  and  the  revolving  lawn  or  rotary 
sieve.  The  two  last  named  are  self-acting  as  regards  removal 


CERAMIC  INDUSTRIES 


187 


of  the  dirt  and  other  rubbish,  whereas  the  three-decked  lawns 
need  to  have  the  rubbish  removed  at  frequent  intervals. 

The  sieved  slip  is  passed  over  a  series  of  magnets  to 
remove  any  specks  of  iron  or  magnetic  iron  compounds. 
Commonly  horseshoe  magnets  are  fixed  in  the  trough 
through  which  the  slip  flows  to  the  stock  ark,  but  sometimes 
special  contrivances  are  used,  often  with  electro-magnets. 

The  prepared  slip  was  formerly  con- 
verted into  clay  suitable  for  use  by  the 
potters  by  running  it  to  a  depth  of  6  to 
8  ins.  over  shallow  drying  kilns  ("  slip 
kilns  ")  made  of  brickwork  or  fireclay 
quarries  or  tiles  with  fires  burning 
beneath.  The  fireplace  is  at  one  end, 
and  the  products  of  combustion  pass 
along  a  flue  to  a  chimney  at  the  otner 
end.  In  this  way  the  water  is  slowly 
removed  by  evaporation,  and  the  clay 
is  obtained  in  workable  condition.  This 
method  is  now  only  used  for  compara- 
tively small  quantities  of  the  clay 
mixtures,  partly  because  of  its  slowness 
and  the  expense  for  fuel,  partly  because 
it  requires  very  careful  beating  to  make 
it  as  homogeneous  as  it  should  be, 
especially  in  view  of  the  possible  presence 
of  hard  lumps  of  clay  which  had  been 
against  the  hot  brickwork.  The  second 
method,  now  in  general  use  except  for 
small  quantities  of  material,  is  to  remove  the  excess  of 
water  by  means  of  a  filter  press.  A  third  method  sometimes 
convenient  for  very  small  quantities  of  material,  but  much 
too  slow  and  expensive  for  use  on  a  large  scale,  is  to  pour 
the  slip  into  plaster  moulds,  when  the  water  is  gradually 
absorbed  by  the  plaster. 

The  filter  press  was  invented  by  William  Needham  and 
James  Kite  (Eng.  pats.  No.  1669,  July  14, 1853,  and  No.  1288, 
1856  ;  also  Kite,  No.  1300,  June  15,  1854).  It  consists 


FIG.  13. — Patent  auto- 
matic deadweight 
pump  (for  clay  slip). 
(Gosling  &  Gatens- 
bury.) 


i88 


SILICA   AND   THE  SILICATES 


of  a  number  of  strong  trays  standing  on  edge  side  by  side. 
These  trays  are  grooved,  and  when  placed  together  each 
adjoining  pair  encloses  a  space  about  an  inch  across.  Within 
these  spaces  are  arranged  strong  cotton  press  cloths,  which 
are  placed  on  the  trays  and  carefully  doubled  over,  the 
edges  overlapping  the  bottoms  and  sides  of  the  trays  being 
folded  up  flat,  so  that  each  cloth  forms  a  kind  of  bag.  It  is 
preferable  to  have  two  cloths,  of  which  the  outer  is  much 
coarser  and  merely  serves  to  protect  the  inner ;  the  two 
cloths  are  folded  together  in  the  tray.  Near  the  middle  of 
the  cloth,  at  the  point  where  it  is  first  doubled,  is  fitted  a 
tube  of  gun  metal,  through  which  the  clay  slip  finds  its  way 


FIG.  I4. — iron  filter  press.     (William  Boulton,  Ltd.) 

into  the  press.  A  stand-pipe,  in  communication  with  the 
supply -pipe  from  the  press  pump,  passes  along  above  the 
press,  and  nozzles  fixed  to  this  stand-pipe  are  connected 
with  the  tubes  from  the  press  cloths.  Each  nozzle  has  a 
tap,  so  that  any  chamber  can  be  disconnected  in  case  a 
cloth  bursts.  The  clay  slip  being  pumped  with  considerable 
force  into  the  cloth  bags,  the  water  passes  away  through 
the  cloth,  and  the  clay  remains.  Pumping  is  then  stopped, 
the  nozzles  are  disconnected,  the  press  taken  apart,  the 
cloths  unfolded,  and  the  clay  taken  out.  The  cloths  are 
folded  again  immediately  for  another  operation  of  the  press. 
The  corrugated  thick  sheets  or  cakes  of  clay  are  rolled 


CERAMIC  INDUSTRIES 


189 


up  and  removed.  The  outer  portions  are  always  harder 
than  the  inside  parts.  The  cloths  need  washing  from  time 
to  time  to  keep  the  pores  open,  for  if  the  water  cannot 
escape  readily  the  cloth  may  be  burst.  With  reasonable 
treatment  a  press  cloth  ought  to  last  four  months  or  more. 
As  the  cloths  contract  when  wet,  they  should  be  wet  when 
folded  in  the  trays,  otherwise  there  is  great  risk  of  bursting. 
The  trays  and  most  other  parts  of  clay  presses  were  formerly 
made  of  wood,  but  there  is  a  growing  tendency  to  use  iron 
presses  because  of  their  strength  and  greater  regularity 
through  absence  of  warping,  etc.  The  only  serious  objection 
to  iron  presses  is  the  risk  of  contaminating  the  clay,  but  with 


FIG.  15. — Pug  mill.     (William  Boulton,  Ltd.) 

proper  working  this  danger  can  be  reduced  to  very  narrow 
limits. 

The  clay  from  either  the  clay  press  or  the  slip  kiln  will 
rarely  be  of  uniform  consistency  and  free  from  air  bubbles. 
This  was  formerly  remedied  by  "  wedging  "  the  clay,  that 
is,  repeatedly  cutting  off  a  piece  of  clay  with  a  wire  and 
dashing  it  with  great  force  against  another  piece  placed  on 
a  block  with  plaster  surface.  The  same  result  is  now 
generally  obtained  by  passing  the  clay  through  a  pug  mill. 
The  essential  parts  of  a  pug  mill  are  a  cylindrical  passage 
traversed  by  an  axle  to  which  are  fixed  iron  blades  set  in 
such  directions  that  they  not  only  cut  the  clay,  but  they 
are  always  forcing  it  onward.  The  clay  is  charged  into  one 
end  of  the  pug  mill,  and  at  the  other  end  it  is  forced  out  as 


igo  SILICA  AND  THE  SILICATES 

a  solid  block  (usually  square  or  rectangular  in  section) 
through  an  aperture  at  the  other  end,  generally  placed 
laterally  (not  terminally),  at  least  in  the  ordinary  horizontal 
pug  mills.  Some  pug  mills  act  vertically,  notably  those 
used  for  china  bodies.  The  clay  as  it  comes  out  is  cut  with 
wire  into  suitable  lengths,  and  afterwards  distributed  to 
the  workpeople,  except  when  required  for  preparing  casting 
slip. 

Ageing. — It  is  often  found  advisable  to  set  the  prepared 
clay  aside  for  "ageing"  or  "rotting."  It  has  long  been 
known  that  ceramic  bodies  become  more  plastic,  more 
solid,  and  better  as  regards  working  qualities,  by  keeping 
them  some  tune  after  being  made,  especially  with  occasional 
watering,  beating,  and  exposure  of  fresh  portions.  Generally 
gases  are  given  off  (usually  carbon  dioxide  and  sulphuretted 
hydrogen),  no  doubt  due  to  gradual  oxidation,  promoted 
by  the  presence  of  air  and  water,  of  carbonaceous  matter 
and  iron  pyrites  nearly  always  occurring  in  plastic  clays. 
The  change  is  probably  for  the  most  part  a  physical  change 
due  to  shrinkage  and  gradual  settling  of  the  particles  which 
accompanies  the  evaporation  of  the  water.  This  is  indicated 
by  the  fact  that  a  clay  body  made  from  "  cuttings  "  is 
always  more  plastic  than  freshly-made  body,  as  already 
mentioned.  Advantage  is  taken  of  this  fact,  in  the  case  of 
certain  bodies  having  only  slight  plasticity,  to  improve 
that  quality  by  partly  shaping  the  material  on  the  potter's 
wheel,  then  shaving  it  down  on  the  lathe  and  making  up 
again,  these  operations  being  repeated  several  tunes. 

Shaping. — The  method  employed  for  shaping  articles 
depends  on  a  variety  of  circumstances,  such  as  the  nature 
of  the  clay,  the  size  of  the  piece  to  be  made,  and  the  shape 
of  the  piece.  When  the  clay  body  is  very  plastic,  and 
articles  of  rounded  form  are  required,  the  method  of  throw- 
ing on  the  wheel  is  very  suitable.  For  other  shapes  (oval, 
angular,  or  irregular),  and  for  all  shapes  when  the  clay  body 
is  not  sufficiently  plastic,  methods  such  as  pressing  or 
casting  must  be  used. 

Throwing. — The  use  of  the  potter's  wheel  dates  from 


CERAMIC  INDUSTRIES 


191 


very  remote  times.  The  simplest  (and  probably  the  earliest) 
form  of  it  is  a  disc  of  wood  fixed  on  the  top  of  a  vertical 
spindle,  which  can  be  rotated  by  hand  by  the  potter  himself 
— known  as  a  "  thrower " — or  by  an  attendant.  The 
addition  of  a  wooden  wheel  attached  to  the  lower  end  of 
the  spindle  enabled  the  rotation  to  be  produced  by  working 
with  the  foot  or  a  stick  on  this  lower  wheel.  A  subsequent 
development  was  the  introduction  of  an  independent  large 
wheel  which  an  attendant  turned  round  by  means  of  a  handle, 
and  the  motion  was  communicated  by  a  cord  or  strap  passing 
round  this  large  wheel,  and  also  round  a  small  pulley  fixed 
on  the  spindle  to  which  the  disc  was  attached.  In  modern 
times  the  potter's  wheel  has  been  turned  by  water  power  or 


FIG.  1 6. — Hand-power  throwing  wheel.     (William  Boulton,  Ltd.) 


steam  power,  and^by  means  of  cones  and  brakes  the  speed  of 
rotation  can  be  regulated  to  meet  the  thrower's  requirements. 
The  thrower  takes  a  ball  of  prepared  clay  of  the  proper 
weight,  throws  it  on  the  rotating  disc,  and  works  it  up  and 
down  with  his  fingers  until  it  is  quite  uniform  in  texture 
and  freed  from  air  bubbles.  He  then  places  his  thumbs  in 
the  middle  of  the  clay,  and  by  pressure  between  thumbs  and 
fingers  gradually  works  the  piece  into  the  correct  shape. 
During  this  operation,  his  ringers  and  thumbs  are  kept  moist 
by  dipping  in  water  at  intervals,  partly  to  prevent  the  clay 
from  sticking  to  the  skin,  and  partly  to  prevent  a  partial 
drying  of  surface  parts  of  the  clay  through  contact  with  the 
warm  hand.  The  finished  piece  is  cut  off  by  a  wire  from 
the  disc,  and  left  in  the  drying  stove  until  it  is  stiff  enough 


192 


SILICA   AND  THE  SILICATES 


for  the  turner  (except  in  rare  cases  in  which  it  is  finished  by 
the  thrower  himself). 

In  recent  times  the  thrown  and  turned  ware  has  greatly 
lessened  in  amount,  the  great  bulk  of  articles  other  than 
artistic  pieces  being  now  made  by  machinery. 

The  turner  finishes  the  shaping  begun  by  the  thrower, 
using  mostly  a  horizontal  lathe  such  as  is  used  for  wood 
and  metals,  etc.  The  excess  clay  should  be  turned  off  in 


n 


FIG.  17. — Power-driven  potter's  lathe.     (William  Boulton,  Ltd.) 

shavings  and  not  in  dust,  for  in  the  latter  case  it  would  be 
almost  certain  to  split  before  being  finished.  Artistic 
pieces  are  turned  on  a  vertical  lathe. 

Pressing. — This  is  the  operation  of  pressing  clay  on  or 
into  moulds,  so  that  the  clay  takes  the  form  of  the  mould. 
The  presser  formerly  made  most  of  the  ware  that  could 
not  be  made  with  machines,  as  ewers,  soup-tureens,  sauce 
boats,  teapots,  etc.,  and  the  process  may  be  said  to  be  older 


CERAMIC  INDUSTRIES  193 

than  any  other  modern  shaping  process  except  throwing. 
In  ordinary  or  plastic  pressing,  the  clay  is  first  flattened  into 
a  thin  cake  or  bat,  on  a  square  plaster  block,  by  beating 
it  with  a  round  plaster  block  to  which  a  handle  is  attached. 
Sometimes  batting  machines  are  used  for  preparing  large 
bats.  The  moulds,  also  usually  of  plaster,  are  mostly 
made  of  several  parts.  Bach  part  of  the  mould  receives 
a  bat  of  clay,  which  is  well  rubbed  down  into  it,  the  different 
parts  are  fitted  together  and  tightly  strapped,  and  the 
joints  (or  seams)  are  then  well  rubbed  so  that  the  clay 
shall  form  a  single  piece.  When  left  to  stand,  the  clay 
begins  to  dry,  at  the  same  time  contracting  slightly  and 
becoming  freed  from  the  mould.  When  the  clay  is  firm 
enough  to  be  handled,  the  parts  of  the  mould  are  removed, 
the  clay  piece  has  its  seams  and  edges  trimmed  and  sponged 
smooth,  and  finally  the  clay  article  is  dried  in  a  stove  before 
being  fired.  Moulds  of  pitcher  (fired  clay),  metal,  sulphur, 
wax,  etc.,  haye  been  used. 

Casting. — This  method  is  used  mainly  with  body  com- 
positions which  are  not  very  plastic,  as  in  the  case  of  ordinary 
porcelain  and  parian  ;  it  is  also  used  for  pieces  which  cannot 
be  properly  pressed.  The  prepared  slip  is  not  in  these  cases 
necessarily  obtained  in  the  stiff  plastic  condition,  but  is 
concentrated  so  as  to  reduce  the  proportion  of  water.  This 
concentration  was  formerly  effected  either  by  avoiding  the 
addition  of  too  much  water  or  by  letting  the  slip  settle  for 
a  time,  and  drawing  off  the  excess  ot  water  from  the  top. 
This  procedure  is  now  obviated  largely  by  the  use  of  certain 
alkaline  substances  in  small  proportions,  those  in  practical 
use  being  sodium  carbonate  (soda  ash),  or  sodium  silicate 
(ordinary  water-glass),  or  preferably  both  together.  Caustic 
soda  would  also  be  effective  in  keeping  the  slip  heavy  without 
sacrificing  fluidity,  but  it  is  not  so  convenient  in  use  because 
of  its  strong  corrosive  action  on  the  skin.  The  proportion 
of  alkali  to  be  used  depends  on  the  nature  of  the  material, 
but  the  more  plastic  the  material  the  more  alkali  is  necessary 
to  maintain  the  same  degree  of  fluidity,  or  a  given  pint  weight 
of  slip,  and  the  less  readily  does  the  piece  come  away  from 
C.  13 


194  SILICA   AND   THE  SILICATES 

the  mould.  In  case  of  trials  where  there  is  nothing  to 
indicate  the  amount  of  alkali  needed,  a  convenient  starting 
point  would  be  about  \  of  i  per  cent,  (half  sodium 
carbonate  and  half  sodium  silicate  or  water-glass)  of  the 
dry  weight  of  the  body. 

It  is  advantageous  in  using  very  plastic  bodies  for  casting 
to  either  substitute  calcined  clay  (or  kaolin)  for  part  of  the 
corresponding  raw  material,  or  to  add  10  per  cent,  (or  even 
more)  of  ground  pitchers.  Some  earthenware  manufacturers 
have  adde(^  ground  pitchers  to  their  ordinary  casting  slip 
for  some  considerable  time  past. 

The  casting  slip  is  poured  into  plaster  moulds,  which 
are  then  left  to  stand  until  the  absorption  of  water  by  the 
porous  mould  has  produced  a  sufficiently  thick  coating 
inside  the  mould.  The  remaining  fluid  slip  is  then  poured 
off,  the  mould  along  with  the  clay  cast  being  left  to  drain, 
after  which  it  is  transferred  to  a  drying  stove.  The  clay 
cast  shrinks  away  from  the  mould  in  drying,  and  can  then 
be  removed  from  it,  trimmed  up,  and  thoroughly  dried 
preparatory  to  firing  it.  Addition  of  a  little  salt  or  soda 
to  the  slip  facilitates  quicker  delivery  of  the  cast  from  the 
mould,  but  this  would  be  very  destructive  to  the  moulds. 

Casting  is  specially  suitable  for  very  thin  ware.  It  also 
gives  more  uniform  thickness  to  the  piece  of  ware  than  could 
be  got  by  pressing,  and  gives  a  very  clear  impression  of  the 
mould — especially  of  elaborate  details ;  the  contraction  is 
also  uniform,  so  there  is  less  liability  to  splitting  in  the 
firing.  A  disadvantage  is  that  as  the  body  after  casting  is 
very  porous,  cast  ware  is  more  liable  to  craze  (or  form  fine 
cracks  in  the  glaze)  than  ware  made  in  other  ways.  Greater 
drawbacks  of  cast  ware  are  the  excessive  contraction  it 
undergoes,  and  the  great  number  of  moulds  required. 

Handles  of  mugs,  chambers,  etc.,  are  made  separately 
by  forcing  clay  out  in  long  strips  from  a  wad  box  (or 
squeezing  box)  through  a  hole  in  a  metal  plate,  which  gives 
the  clay  the  desired  shape  ;  the  strips  are  cut  off  in  suitable 
lengths,  bent  into  the  proper  shape,  the  ends  being  then 
trimmed,  and  the  handles  fixed  by  means  of  a  little  slip 


CERAMIC  INDUSTRIES  195 

on  the  mugs  or  other  articles.  For  the  more  ornamental 
handles  small  round  rolls  of  clay  are  first  produced  with  the 
aid  of  the  wad  box  ;  these  rolls  are  cut  into  suitable  lengths, 
one  of  which  is  placed  in  one-half  of  a  two-piece  plaster 
handle  mould,  the  other  half  being  then  fitted  on,  and  the 
two  halves  pressed  well  together.  The  handle  is  removed 
after  a  short  time,  and  placed  on  a  board  to  dry,  after  which 
it  is  trimmed  and  fixed  in  its  place  by  slip.  In  a  similar 
way  spouts  are  fixed  on  teapots,  feet  on  soup-tureens, 
knobs  on  dish  covers,  etc.  Pieces  which  are  made  in  several 
parts  are  also  stuck  together  in  much  the  same  way.  Certain 
of  these  articles,  teapots,  for  instance,  are  sometimes  made 
complete  by  casting,  special  contrivances  being  employed. 

Most  dishes  which  are  not  round  (but  oval,  etc.)  are  made 
by  hand,  a  prepared  bat  of  clay  being  carefully  pressed  on  a 
mould ;  the  mould  is  then  placed  on  a  whirler  (or  round 
block  of  plaster  fixed  at  the  top  of  a  spindle),  and  the  inside 
surface  of  the  clay  smoothed  carefully.  When  dry  enough, 
the  dishes  are  fettled,  and  then  placed  face  downwards  on 
perfectly  flat  plaster  slabs,  so  that  they  cannot  go  crooked 
in  further  drying. 

The  processes  of  throwing,  pressing,  and  casting,  subject 
to  slight  differences  in  detail  when  working  with  different 
bodies,  are  essentially  all  that  are  used  for  making  the 
various  types  of  pottery.  For  the  purpose  of  saving  labour 
much  machinery  has  been  introduced,  though  very  strong 
opposition  was  offered  by  the  workmen  at  the  outset. 

In  pottery,  as  in  other  manufacturing  industries,  pro- 
duction is  much  increased  by  the  proper  application  of 
suitable  machinery.  The  motive  power  is  largely  derived 
from  steam  or  water  power,  which  is  transmitted  by  shafting 
in  the  case  of  the  heavy  machinery  of  the  slip-house  and 
grinding  mills,  and  by  ropes  or  belts  for  the  machines  con- 
cerned in  the  making  of  the  ware.  In  recent  years  electric 
motors  have  been  increasingly  brought  into  use  in  pottery 
works,  especially  for  situations  remote  from  the  boiler. 
Rotary  motion  is  produced  in  nearly  all  the  machines,  generally 
by  means  of  a  spindle,  of  which  the  lower  end  works  in  a 


196  SILICA   AND   THE  SILICATES 

socket,  and  the  upper  part  in  some  kind  of  collar  ;  a  grooved 
wheel  is  fixed  to  the  spindle,  so  that  it  can  be  turned  by  a 
running  rope.  The  top  of  the  spindle  ends  in  a  screw,  on 
which  heads  or  other  appliances  can  be  placed. 

For  making  flat  and  hollow  ware,  an  automatic  batting 
machine  is  used,  which  has  a  spindle  such  as  that  just 
referred  to,  with  a  circular  plaster  block  fixed  (by  means  of 
an  iron  part)  on  the  screw  forming  the  top  end  of  the  spindle. 
A  fixed  iron  upright  supports  an  arm  which  works  up  and 
down  on  a  pivot.  This  arm  carries  at  one  end  a  tool  or 
spreader,  which  is  a  flat  piece  of  iron,  one  end  of  which  is 
just  over  the  middle  of  the  plaster  block.  The  tool  varies 
in  size  according  to  the  size  of  bat  required.  An  iron  rod 
connects  the  other  end  of  the  arm  with  a  lever  attached  to 
a  cog-wheel  which  is  moved  by  means  of  a  screw  on  the 
spindle.  Thus  movement  of  the  spindle  sets  the  cog-wheel 
going,  and  that  pushes  up  the  lever  and  the  arm,  and  so 
brings  the  tool  down  to  or  towards  the  plaster  block.  The 
workman  places  a  lump  of  clay  in  the  middle  of  the  plaster 
block,  and  then  starts  the  machine  by  setting  the  spindle 
in  rotation.  As  the  tool  descends,  the  clay  is  spread  out 
in  a  round  flat  bat  of  the  thickness  determined  by  the  setting 
of  the  tool.  On  the  tool  reaching  its  lowest  point,  the  lever 
is  automatically  released,  the  tool  rises  again,  and  the 
machine  stops.  In  the  absence  of  the  machine  the  workman 
would  have  to  beat  out  every  clay  bat  by  hand  with  a  batter 
on  a  plaster  block. 

Jolleys  (or  plate  machines)  are  special  machines  for 
making  plates,  saucers,  bowls,  etc.,  a  different  tool  being 
used  for  each  different  size  and  shape.  Bach  machine 
has  a  frame  and  bed  fixed  to  a  working  bench,  and  has  also 
a  spindle  ending  above  in  a  screw  by  which  can  be  adjusted 
various  heads  into  which  moulds  are  made  to  fit.  When 
in  use,  a  mould  is  firmly  fixed  in  the  head,  and  should  run 
quite  true  when  the  spindle  is  turned.  Fixed  to  the  bench 
(or  bed)  behind  the  spindle  is  an  upright  support,  which 
works  in  a  socket,  and  is  kept  at  the  proper  height  by  a 
set  pin.  The  upright  support  carries  an  arm  and  handle, 


CERAMIC  INDUSTRIES 


197 


capable  of  moving  only  vertically  (up  or  down)  ;  the  exact 
position  of  the  lower  end  of  the  arm  is  regulated  by  a  screw, 
and  the  tool-holders  are  firmly  attached  to  this  arm.  A 
tool  or  profile  is  fixed  to  the  tool-holder  in  the  required 
position,  and  in  working  is  held  down  by  the  handle  con- 
nected to  the  arm.  The  other  end  of  the  arm  (behind  the 
upright)  has  a  counter-weight  which  causes  the  tool-holder 
and  tool  to  swing  up  when  not  held  down  by  the  workman. 

In  plates,  saucers,  and  other  flat  articles,  the  mould 
forms  the  face  or  normally  upper  surface,  and  the  profile 
or  tool  forms  the  outside  or  back  of  the  piece.  On  the  other 
hand,  in  hollow-ware 
(cups,  bowls,  chambers, 
soup-tureens,  etc.),  the 
mould  forms  the  outside, 
and  the  tool  gives  the 
inside  shape.  Special 
modifications  are  used  in 
making  certain  articles, 
and  some  of  them  have 
special  names — such  as 
the  Monkey  or  Upright 
Jolley  (for  making 
chambers,  jugs,  jars,  and 
pieces  with  large  bodies 
and  small  mouths),  the 
Automatic  Cup  -  making 


0 

- 

A 

5=       = 

E= 

, 

FIG.  1 8. — Rope-driven  jigger  and  jolley 
(William  Boulton,  Ltd.) 


Machine,  the  Automatic  Chamber  and  Basin  Machine,  etc. 

Machines  were  introduced  for  making  earthenware 
about  eighty  years  ago,  and  the  innovation  met  with  such 
determined  opposition  from  the  workmen  that  considerable 
delay  took  place  before  the  regular  employment  of  machines 
for  the  purpose  got  well  established.  (See  Eng.  pats.  8339 
and  8340,  January  n,  1840,  John  Ridgway  and  George 
Wall.  Also  9901,  October  5,  1843,  George  Wall.) 

Drying  Stoves  are  used  for  removing  as  much  as 
possible  of  the  moisture  before  firing  the  ware.  In  some 
lands  the  sun's  heat  can  be  utilized  for  the  purpose,  but  in 


198  SILICA   AND   THE  SILICATES 

this  country  at  least  that  would  not  be  practicable.  The 
plaster  moulds  also  need  drying  periodically  in  order  that 
they  may  be  worked  to  their  full  capacity.  The  stoves 
should  be  arranged  so  that  the  carrying  to  and  fro  of  the 
ware  shall  be  reduced  to  a  minimum.  Each  stove  is  con- 
structed about  a  central  upright  shaft  resting  in  a  socket 
below  and  in  a  bearing  above.  The  shaft  supports  iron 
arms  and  ties  forming  a  frame  which  divides  the  stove  into 
compartments  (mostly  six).  Each  compartment  is  occupied 
by  shelves  on  which  the  ware  and  moulds  are  placed,  and 
the  doors  are  made  of  the  width  of  a  compartment,  so  that 
as  the  frame  is  revolved  the  compartments  are  in  turn 
brought  opposite  the  door  for  the  removal  and  introduction 
of  moulds  and  ware.  The  size  of  the  stoves,  and  especially 
the  number  of  shelves,  depends  to  some  extent  on  the  class 
of  ware  for  which  they  are  intended ;  a  common  size  is 
about  12  ft.  across  and  10  or  12  ft.  high.  The  stoves  are 
usually  heated  by  beds  of  steam  pipes,  the  exhaust  steam 
being  used  for  the  purpose.  Sometimes  hot  air  or  waste 
gases  from  kilns  or  ovens  are  used  in  like  manner ;  this  is 
a  common  method  in  Germany.  The  drying  is  greatly 
facilitated  by  ventilation  at  the  top  of  the  stove  to  enable 
the  moisture  to  get  away  readily.  For  large  hollow-ware, 
the  revolving  stove  is  not  suitable,  shelving  round  the  stove 
room  being  preferable  ;  the  heating  is  effected  in  the  same 
way.  In  a  recent  American  invention,  the  ware  travels 
through  the  stove,  alternately  upwards  and  downwards  in 
passing  along.  Two  endless  sprocket  chains  are  connected 
with  a  series  of  sprocket  wheels,  and  boards  to  support  the 
ware  are  suspended  from  the  chains.  It  is  very  efficient  and 
very  adaptable.  (See  Trans.  Ceramic  Society,  ip,  26,  1920.) 

After  the  ware  has  been  dried,  it  is  fettled  and  cleaned, 
and  the  edges  of  plates  are  polished.  This  is  done  on  a 
whirler  with  a  plaster  head  such  as  is  used  by  a  presser, 
but  smaller  and  lighter.  Very  simple  tools  are  needed. 

The  ware  is  then  ready  to  be  fired,  unless  decoration  in 
the  clay  state  is  intended. 

Decoration  in  the  clay  state  is  effected,  before  the  ware 


CERAMIC  INDUSTRIES  199 

is  too  dry,  by  applying  figures,  flowers,  etc.  (previously 
made  in  moulds)  to  the  surface  of  the  piece,  fixing  them  by 
means  of  slip  ;  by  pressing  (stamping)  small  dies,  etc.,  on 
the  ware  ;  by  modelling  with  the  hands ;  by  building  up 
(as  in  the  case  of  basket  work  or  flowers)  ;  by  perforation  ; 
by  marking  with  scratched  lines,  dots,  etc.,  or  by  the  use 
of  coloured  slips,  or  by  both  these  means  jointly.  Decoration 
by  scratched  lines  is  termed  "sgraffito."  Coloured  bodies 
have  also  been  used,  the  colour  arising  from  the  presence  of 
certain  mineral  substances,  mainly  colouring  metallic  oxides. 
Such  colouring  of  the  body  is  applicable  to  all  the  best 
ceramic  bodies,  such  as  fine  earthenware,  porcelain,  jasper, 
etc. 

During  the  drying  of  clay  ware,  contraction  or  shrinkage 
takes  place.  A  slight  contraction  takes  place  in  the  mould 
because  of  absorption  of  some  moisture  by  the  plaster. 
After  the  piece  of  ware  has  been  removed  (still  somewhat 
moist)  from  the  mould,  it  gradually  loses  all  the  water 
added  to  the  materials  for  mixing  purposes,  etc.,  and  also 
part  of  the  moisture  originally  present  in  them,  especially 
in  the  clays.  As  the  water  passes  away,  the  solid  particles 
draw  closer  and  closer  together.  During  the  subsequent 
firing,  all  the  remaining  water  is  driven  out — slowly  and 
very  gradually  to  avoid  cracking — including  the  combined 
water  in  clays,  and  further  contraction  takes  place  in  a 
similar  way.  After  that,  the  particles  begin  to  enter  into 
chemical  reaction,  and  the  more  fusible  components  become 
vitrified,  and  these  changes  are  accompanied  by  a  different 
kind  of  contraction,  the  amount  of  which  increases  as  the 
clay  body  becomes  more  vitreous. 

In  earthenware  bodies  the  proportion  of  fusible  material 
is  very  low,  so  the  contraction  in  such  bodies  during  the 
finishing  stage  of  firing  is  very  low  in  comparison  with  that 
of  vitreous  bodies.  The  contraction  in  earthenware  bodies 
is  commonly  about  8  per  cent.,  or  i  in  12,  but  will  vary 
according  to  the  composition  of  the  body  (especially  as 
regards  the  fusible  constituents),  and  the  temperature  and 
duration  of  the  firing.  The  contraction  is  also  dependent 


200  SILICA   AND   THE  SILICATES 

on  the  method  of  manufacture,  increased  pressure  being 
associated  with  reduced  contraction.  Cast  ware  contracts 
most,  thrown  and  turned  ware  contract  more  than  pressed 
ware,  and  tiles  and  other  articles  made  with  metal  dies 
under  great  pressure  will  contract  least.  Hence  special 
care  is  necessary  in  uniting  parts  made  by  different  methods, 
particularly  when  the  clay  is  very  plastic.  The  same  thing 
— difference  of  pressure — accounts  for  the  fact  that  seams 
of  a  piece  (where  the  parts  of  a  mould  fitted  together)  are 
always  evident. 

Sometimes — especially  in  tall  pieces  and  in  cast  ware — 
contraction  is  greater  vertically  than  horizontally,  doubtless 
partly  owing  to  the  compressive  action  of  the  weight  of  the 
upper  part  on  the  lower  part. 

FIRING  WARE. 

The  firing  may  be  said  to  be  the  most  important  operation 
of  all,  for  nothing  done  in  other  operations  can  make  up  for 
unsatisfactory  firing.  It  is  equally  true  that  for  satisfactory 
results  good  firing  is  necessary  for  glost  (that  is,  glazed) 
ware  as  for  the  first  or  biscuit  ware.  The  general  type  of 
oven  used  for  firing  earthenware  is  circular  in  section,  with 
a  dome  or  crown  on  the  top.  All  consist  essentially  of 
mouths  (fire-places),  flues,  and  the  single  large  firing  chamber, 
but  with  differences  in  different  parts  of  the  oven.  The 
mouths  are  arranged  regularly  round  the  outside  of  the  oven, 
near  the  bottom.  In  the  old-fashioned  Hob-mouthed 
Ovens,  which  are  still  much  used,  the  mouths  project  out- 
wards (i|  to  2  ft.)  from  the  sides,  coal  being  fed  in  through  a 
hole  in  the  top,  which  can  be  covered  with  a  slab  of  fireclay. 
The  stack  generally  springs  from  the  shoulder  of  the  oven. 
It  is  assumed  that  cold  air  cannot  easily  be  drawn  into  the 
flues  while  "  baiting  "  (feeding  with  coal).  In  Hovel  Ovens, 
the  oven  proper  is  surrounded  by  a  circular  hovel,  which 
rises  vertically  to  about  the  level  of  the  shoulder  of  the  oven, 
and  then  gradually  narrows  in  the  same  way  as  the  shoulder 
and  neck  of  a  bottle,  the  neck  forming  the  stack  which  carries 


CERAMIC  INDUSTRIES  201 

off  the  smoke,  etc.  The  space  between  oven  and  hovel  is 
barely  enough  for  the  distribution  of  the  coal.  These  ovens 
have  mouths  protected  from  air  currents,  and  repairs  to 
the  oven  can  easily  be  made,  but  the  space  round  the  oven 
is  very  limited  for  firing,  and  the  draught  is  sometimes 
imperfect. 

Stack  Ovens  have  the  ovens  and  stacks  built  together. 
They  are  solid  and  compact,  but  when  the  mouths  are  first 
lighted  they  are  liable  to  smoke,  and  deposit  dirt  in  the  adjoin- 
ing shops.  They  are  also  somewhat  more  difficult  to  repair, 
and  are  slow  in  cooling.  Down-draught  Ovens  are  much 
used  for  biscuit  firing,  and  for  firing  firebricks.  They  can 
be  worked  with  less  fuel,  and  with  better  distribution  of 
heat,  owing  to  special  arrangements  of  flues.  Several  kinds 
of  down-draught  oven  have  been  extensively  used.  In 
the  Up -draught  Ovens,  which  are  still  mostly  used  for  glost 
firing,  the  flames  and  gases  from  the  mouths  pass  partly 
along  horizontal  flues  below  the  floor  of  the  oven  to  the 
central  well-hole,  from  which  they  pass  upwards  through 
the  middle  of  the  firing  chamber,  and  partly  up  short 
chimneys  (called  "  bags  ")  just  inside  the  oven,  and  thence 
to  the  top  of  the  oven.  A  pipe  bung  of  fireclay  rings,  or 
saggars  with  holes  in  the  bottoms,  is  often  built  up  from 
over  the  well-hole  to  near  the  top  of  the  oven.  The  crown 
has  a  central  hole,  controlled  by  a  damper,  which  regulates 
the  discharge  of  gases,  etc. 

In  Robey's  Down-draught  Oven  (patented  by  C.  Robey, 
B.  Banks,  and  T.  Forester,  No.  970,  March  15,  1873),  flues 
pass  under  the  floor  from  each  mouth  to  a  central  well- 
hole,  and  between  each  pair  of  these  flues  a  passage  runs 
through  the  floor  of  the  oven  down  into  a  vaulted  chamber 
below.  This  vaulted  chamber  is  connected  by  a  flue  with 
the  chimney  stack,  which  either  passes  up  the  outside  of 
the  oven,  or  is  quite  separate  from  it  and  communicates 
with  several  ovens.  On  commencing  to  fire  the  oven,  the 
damper  on  the  crown-hole  is  opened,  and  the  chimney 
flue  shut  off,  but  after  the  fires  are  well  started  the  damper 
is  gradually  closed  and  the  chimney  connected.  This  oven 


202  SILICA   AND   THE  SILICATES 

is  economical  in  fuel,  but  the  distribution  of  the  heat  is  by 
no  means  perfect. 

Minton's  Down-draught  Oven  (T.  W.  Minton,  No.  1709, 
May  10,  1873)  has  very  much  thicker  walls  than  those  of 
ordinary  up-draught  ovens,  and  the  mouths,  which  are  only 
half  the  usual  size  (though  slightly  taller  and  narrower) 
and  are  built  entirely  in  the  walls,  open  into  the  interior  of 
the  oven  only  through  bags.  The  only  flues  communicating 
with  the  well-hole  are  horizontal  g-in.  flues  passing  under 
the  flat  floor  to  vertical  flues  situated  in  the  oven  walls 
between  the  bags.  These  vertical  flues  run  up  above  the 
shoulder  of  the  oven  and  open  into  a  hood  or  hovel  built 
on  the  top  of  the  oven,  and  forming  an  upper  chamber 
where  printed  ware  can  be  hardened  on,  or  certain  kinds  of 
pottery  can  be  fired  at  a  low  temperature.  The  damper 
for  the  crown-hole  is  an  essential  feature  of  the  oven,  but 
there  are  no  clearing  holes  round  the  shoulder  like  those  in 
up-draught  ovens.  When  the  upper  chamber  is  dispensed 
with,  the  vertical  flues  are  omitted,  and  the  gases  then  pass 
on  from  openings  in  the  floor  to  a  central  opening  leading 
to  an  outside  chimney.  The  advantages  of  Minton's  ovens 
are  economy  of  fuel  (often  30  to  40  per  cent.),  better  dis- 
tribution of  the  heat,  and  better  control  of  the  firing.  Dis- 
advantages are  the  greater  cost  (as  much  as  40  per  cent, 
more)  in  building,  greater  wear  and  tear,  and  irregularities 
of  firing  unless  the  flues  and  air-spaces  are  properly  pro- 
portioned. The  greater  wear  and  tear  might  probably  be 
largely  obviated  by  a  suitable  selection  of  refractory  materials 
for  lining  the  flues,  etc.  The  crown-hole  damper  is  raised 
to  promote  cooling  of  the  ware  after  firing  the  oven,  but  if 
raised  too  quickly,  some  of  the  ware  would  certainly  be 
dunted  (that  is,  cracked),  for  dunting  takes  place  between 
red  heat  and  dull  heat. 

Wilkinson's  Down-draught  Oven  (A.  J.  Wilkinson, 

No.  4356,  March  20,  1890)  has  two  sets  of  flues  under  the 

.floor  of  the  oven,  the  products  of  combustion  passing  through 

one  set  to  the  central  well-hole,  and  thence  into  the  oven — 

as  also  another  portion  directly  upwards  through  the  bags — 


CERAMIC  INDUSTRIES  203 

and  the  other  set,  receiving  the  gases,  etc.,  through  openings 
in  the  floor,  and  conducting  them  to  dampered  vertical 
flues  running  in  the  walls  ;  sometimes  the  gases  are  allowed 
to  pass  away  by  openings  in  the  crown.  The  heat  of  the 
oven  is  regulated  by  admission  of  air  above. 

Generally  speaking,  down-draught  ovens  are  found 
advantageous  for  biscuit  firing,  because  of  the  slower, 
steadier,  and  more  uniform  development  of  the  heat,  and 
up -draught  ovens  are  mostly  preferred  for  glost  firing, 
because  of  the  more  rapid  development  of  the  heat  and  the 
more  ready  escape  from  the  oven  of  the  products  of  com- 
bustion and  any  other  fumes — so  that  the  liability  to 
"  sulphuring "  (generally  arising  from  partial  reduction 
of  lead  compounds  in  the  glaze)  is  lessened. 

The  hovel  built  over  (and  in  some  cases  about)  a  pottery 
oven  serves  the  useful  purposes  of  creating  a  draught, 
facilitating  the  escape  of  smoke  and  other  products  of  com- 
bustion, and  screening  the  mouths  from  winds  which  might 
cause  inequalities  of  burning — the  fires  in  some  mouths 
burning  up  fiercely,  whilst  in  those  on  the  opposite  side 
the  flame  might  be  blown  back  out  of  the  oven  mouths. 
When  the  hovel  starts  from  the  shoulder  of  the  oven,  the 
latter  is  more  or  less  surrounded  by  placing  sheds  (or  saggar- 
house)  and  warehouses  which  screen  the  mouths  from  winds. 

The  so-called  Skeleton  Oven,  which  has  come  into 
extensive  use,  apparently  combines  in  itself  the  best  features 
of  the  various  up-draught  ovens,  being  simple  to  work, 
clean,  easy  to  repair,  quick  to  cool,  and  also  giving  satis- 
factory results,  whilst  occupying  a  comparatively  small 
space.  In  a  skeleton  oven  the  stack  is  supported  round 
the  oven  by  a  number  of  arches,  leaving  the  oven  separated 
from  the  shell  or  arches  by  a  few  inches  only  ;  the  first 
three  feet  or  so  (up  to  the  top  of  the  mouths)  of  the  oven 
and  outside  brickwork  are  built  together,  but  above  this 
level  the  oven  stands  quite  free  from  the  shell. 

It  may  be  noted  that  while  very  large  ovens  are  econo- 
mical as  regards  fuel,  great  attention  is  needed  to  get  any- 
thing like  a  regular  distribution  of  heat ;  they  also  take 


204  SILICA   AND  THE  SILICATES 

longer  to  set  and  draw,  and  longer  to  cool ;  ovens  that  can 
be  rapidly  and  progressively  filled,  fired,  and  emptied, 
answer  best  for  trade  purposes. 

Various  other  types  of  oven  have  been  devised  for  firing 
pottery,  including  various  forms  of  gas-fired  ovens,  and  con- 
tinuous ovens,  but  have  been  only  partially  successful  in 
practice,  except  for  comparatively  low  temperatures,  in 
this  country.  They  have  been  much  more  extensively  used 
in  the  United  States,  Germany,  and  France,  and  it  seems 
quite  likely  that  in  the  not  very  distant  future  the  use  of 
some  forms  of  continuous  ovens  and  kilns  may  spread 
considerably,  even  in  the  United  Kingdom. 

In  firing  bricks,  common  flooring  tiles,  some  kinds  of 
terra-cotta  and  stonewares,  the  exclusion  of  smoke  and 
flame  is  not  necessary,  but  for  firing  earthenware,  china, 
parian,  white  stoneware,  or  other  white  or  artificially 
coloured  bodies,  it  is  necessary  to  employ  saggars  to  protect 
the  wares  from  the  direct  action  of  flame,  etc. 

Saggars  are  fireclay  boxes  of  various  sizes  and  shapes, 
but  mostly  round  or  oval.  They  are  made  of  mixtures  of 
fireclay  (commonly  termed  marl)  and  grog,  the  latter  being 
crushed  or  ground  material  from  old  saggars,  firebricks, 
broken  biscuit  ware  (earthenware  or  china),  or  any  refractory 
material. which  owing  to  previous  firing  or  in  other  ways  is 
not  liable  to  any  considerable  amount  of  contraction.  For 
special  purposes  the  ordinary  grog  has  been  advantageously 
substituted  by  comparatively  expensive  materials  such  as 
emery,  corundum,  or  carborundum.  The  most  important 
qualities  of  saggars  are  that  they  should  be  sufficiently 
refractory  to  resist  the  heat  of  the  ovens  without  losing  their 
shape,  and  that  they  should  bear  sudden  changes  of  tem- 
perature without  fracture,  and  should  have  strength  enough 
to  stand  whatever  pressure  they  may  be  subjected  to  in  the 
oven.  These  properties  may  be  secured  by  using  a  fire- 
T-fey  (or  marl)  rich  in  silica,  and  as  free  as  possible  from  iron- 
stone E^dules.  Flying  off  or  scaling  of  bits  of  saggar  may  be 
caused  by  -^articles  of  quartz,  iron  compounds,  or  lime, 
or  by  imperfect  ™ixiT1g-  ^ocal  fireclays  (marls)  with  about 


CERAMIC  INDUSTRIES  205 

55  to  60  per  cent,  of  silica  are  refractory  enough  for  making 
saggars  to  be  used  for  earthenware  and  tile  bodies,  but  for 
the  harder  china  bodies,  etc.,  the  more  refractory  fireclays 
from  the  Stourbridge,  Dudley,  and  Ironbridge  districts  are 
used  for  saggar  making.  As  all  such  fireclays  are  mas- 
sive, and  usually  hard  in  their  natural  condition,  the  marl 
for  saggars  is  first  pulverized  by  passing  through  chilled 
iron  rollers  or  by  grinding  on  a  heavy  roller  pan.  It  should 
then  be  left  to  weather,  and  after  addition  of  the  grog  the 
whole  is  well  mixed  and  turned  over,  then  left  for  some  time 
before  being  used.  Better  mixing  is  secured  by  passing 
it  twice  through  a  pug  mill.  The  coarser  the  fireclay  the 
less  grog  is  required,  and  the  more  plastic  the  fireclay  the 
more  grog  is  required.  The  greater  the  proportion  of  grog 
the  more  open  the  mixture  and  the  better  the  resulting 
saggars,  providing  the  working  qualities  of  the  mixture 
are  not  unduly  lowered.  The  less  water  and  the  stiffer 
the  mixture  the  better  the  saggars,  though  it  is  harder  work 
to  make  them.  Heinecke  states  that  the  best  saggars  are 
obtained  by  mixing  the  dry  clay  and  grog,  and  afterwards 
moistening,  and  that  they  have  been  made  very  successfully 
in  this  way  in  several  German  factories.  As  the  sides  of 
saggars  have  mostly  to  bear  considerable  weights  (due  to 
saggars  above  them  with  contained  ware),  whilst  bottoms 
have  only  to  support  the  ware  actually  in  the  respective 
saggars,  the  bottoms  need  not  have  the  same  strength,  and 
so  the  bottoms  should  have  relatively  more  grog  than  the 
sides.  For  the  sides  6  to  8  cwt.  grog,  and  for  the  bottoms 
10  to  12  cwt.  grog,  to  each  ton  of  fireclay,  are  sometimes 
used.  Some  fireclays  can  be  worked  with  two  parts  only 
to  three  or  more  parts  of  grog.  In  general,  a  larger  pro- 
portion of  grog  is  used  on  the  Continent  and  in  America  than 
in  Great  Britain  ;  from  40  to  60  per  cent,  grog  is  mostly 
used  in  Germany,  the  clay  being  half  fat  and  half  lean. 
Sometimes  5  to  10  pet  cent,  charcoal  powder  or  saw-dust  is 
added.  In  practice  it  is  found  to  be  better  to  use  a  mixture 
of  clays  rather  than  a  single  kind  of  clay.  For  small  saggars 
fine  and  medium  grog  are  most  suitable  (say  1:3),  and  for 


206  SILICA   AND   THE  SILICATES 

larger  saggars  medium  and  coarse  grog  are  best.  Increase 
in  either  size  or  proportion  of  grog  decreases  strength  in 
both  the  green  and  fired  states,  and  also  increases  porosity. 

According  to  F.  A.  Kirkpatrick  (Trans.  Amer.  Cer.  Soc., 
ip,  268,  1917),  with  the  use  of  equal  weights  of  grog  and 
plastic  fireclay  (the  latter  ground  to  pass  a  20-mesh  sieve) 
the  best  mixtures  for  resistance  to  repeated  heating  and 
cooling  contain  grog  of  4-8  mesh,  8-12  mesh,  and  12-20  mesh 
in  the  proportions  1:2:2  and  1:3:1  respectively,  and 
grog  of  8-12  mesh,  12-20  mesh,  and  20-40  mesh  in  the 
proportions  1:1:3  an(i  i  :  i  :  i  respectively  ;  the  best 
mixtuies  for  giving  strength  to  the  saggars  are  grog  of  12-20 
mesh,  20-40  mesh,  and  40-80  mesh  in  the  proportions 
2:1:0,  3:1:1,  2:2:1,  1:2:0,  1:3:1,  0:2:1,  and 
also  the  12-20  mesh  or  20-  40  mesh  alone. 

Saggar  marls  and  bonding  fireclays  fall  into  two  classes, 
vitrifying  clays  (which  vitrify  at  a  comparatively  low  tem- 
perature, and  keep  their  low  porosity  over  a  wide  range 
of  temperature  without  swelling  or  being  overburned), 
and  open-burning  clays  (which  remain  porous  and  highly 
absorbent  up  to  a  comparatively  high  temperature).  With 
proper  size  and  proportioning  of  grog  particles,  proper 
proportioning  also  of  vitrifying  clay  and  open-burning 
clay  (for  combined  strength  and  porosity),  and  with  uniform 
mixing  and  tempering,  saggars  of  the  desired  strength  in 
the  cold  can  be  obtained.  Good  proportions  (recommended 
by  G.  H.  Brown  in  Journal  of  American  Ceramic  Society, 
vol.  2,  1919)  are  50  grog,  20  vitrifying  clay,  and  20  open- 
burning  clay.  lyow-grade  China  clay  would  be  very  good 
for  the  open-burning  clay. 

It  is  good  practice  to  grade  the  grog  particles,  and  to 
include  definite  proportions  of  different  sizes  in  the  mixture 
for  making  saggars,  as  this  gives  a  more  complete  skeleton 
of  grog  particles,  united  by  the  bonding  clay.  Addition  of 
sand  to  the  mixture  makes  the  saggars  more  liable  to  crack, 
since  the  saggar  as  a  whole  may  be  unable  to  resist  the  effects 
of  the  volume  changes  of  the  sand  grains.  For  mixing  and 
tempering  saggar  mixtures,  a  wet  pan  has  been  found  to 


CERAMIC  INDUSTRIES  207 

give  better  results  (in  the  United  States)  than  a  soaking 
pit  followed  by  pug  mill.  The  wet  pan  develops  maximum 
plasticity  of  bond  clays,  so  that  more  grog  can  be  added, 
and  this  involves  less  contraction.  Saggars  made  from 
wet  pan  mixtures  also  mould  better,  and  have  smoother 
surfaces.  With  the  mullers  of  the  wet  pan  slightly  raised 
the  grog  particles  can  slide  under  without  being  crushed. 
The  use  of  a  low-grade  ball  clay  as  bonding  clay  would 
enable  more  grog  to  be  used  in  the  mixture. 

Small  saggars  may  be  made  by  expression  from  a  press 
in  the  same  way  as  ordinary  drain  pipes  are  made.  The 
tubes  so  obtained  are  cut  into  lengths,  and  bottoms  are 
stuck  on  with  clay  slip. 

The  sides  of  saggars  are  commonly  made  by  beating  out 
the  mixture  of  marl  and  grog.  The  flattened  cake  is  cut  up 
into  pieces  of  suitable  size,  and  each  such  piece  is  then  placed 
round  a  wooden  drum  or  mould.  The  drum  with  its  attached 
side  is  then  fitted  to  a  beaten-out  piece  of  mixed  saggar  marl 
for  the  bottom,  and  the  two  are  well  worked  together. 
The  drum  is  then  withdrawn,  and  the  inside  of  the  saggar 
made  good,  after  which  the  saggar  is  taken  away  on  its 
shord  (wooden  support)  to  dry.  The  pieces  for  the  sides 
of  saggars  are  sometimes  prepared  by  pressing  under  rollers. 
Hydraulic  and  other  machinery  have  also  been  employed 
for  making  saggars  complete,  but  have  not  come  into  any- 
thing like  general  use.  The  same  statement  applies  to  cast- 
ing processes.  There  seems  no  valid  reason  why  such 
processes  should  not  be  commercially  successful,  provided 
they  are  properly  carried  out  with  suitable  materials. 

It  may  be  added  that  in  many  factories  the  saggar- 
making  department  receives  less  attention  than  almost 
any  other,  with  the  result  that  enormous  losses  occur.  The 
opinion  seems  to  be  widely  prevalent  that  it  is  not  worth 
serious  attention  or  that  no  great  improvement  is  practicable. 
This  is  a  costly  heresy,  for  great  economies  could  be  effected 
by  competent  supervision.  It  should  be  realized  that  a 
considerable  increase  in  outlay  for  better  raw  materials 
(as,  say,  low-grade  China  clay  and  ball  clay,  or  even  high- 


208  SILICA   AND   THE  SILICATES 

grade  fireclay,  instead  of  common  saggar  marls,  with  care- 
fully selected  material  for  grog)  would  be  handsomely 
compensated  in  the  greatly  extended  average  life  of  the 
saggars,  and  consequent  saving  in  making  costs.  Probably 
the  best  materials  for  general  use  would  be  inferior  ball 
clays  (as  bonding  clays),  inferior  China  clays  (as  open- 
burning  clays),  with  grog  in  the  form  of  crushed  biscuit 
pitchers — either  good  white  earthenware  or  china,  though 
for  china  biscuit  ovens  it  might  be  advisable  to  use  a  more 
refractory  grog  material.  As  an  extreme  case  it  may  be 
recalled  that  K.  Bleod  reported  saggars  cast  from  corundum 
(fused  bauxite)  or  carborundum,  mixed  with  a  little  English 
ball  clay,  as  having  been  repeatedly  raised  to  white  heat 
and  thrown  into  cold  water  without  showing  even  cracks. 

Another  point  to  be  borne  in  mind  in  connection  with 
saggars  is  that  the  thinner  the  walls  and  bottom  the  better, 
providing  they  have  sufficient  strength.  This  gives  advan- 
tage as  regards  both  heading  up  and  cooling,  as  well  as  in 
carrying  by  the  placers.  In  this  connection  it  may  be 
mentioned  that  in  Germany  smaller  saggars  are  much  used, 
and  it  has  been  even  contended  very  plausibly  that  saggars 
for  single  pieces  of  ware  (as  a  teapot,  etc.)  would  be  econo- 
mical as  regards  space  in  the  glost  oven.  The  thin  bottom 
is  often  strengthened  by  two  ribs  crossing  at  right  angles 
in  the  middle  outside. 

Broken  saggars — in  not  more  than  three  pieces — can 
generally  be  profitably  mended  by  using  a  rather  stiff  paste, 
made  with  water-glass  and  water,  of  dust  grog,  alone  or 
mixed  with  some  China  clay  or  fireclay.  The  paste  should 
be  made  as  required,  for  it  soon  sets.  Too  much  raw  clay 
should  not  be  used. 

Placing  or  Setting  is  the  arrangement  of  the  ware, 
usually  in  saggars,  for  firing  in  the  ovens.  The  details 
differ  according  to  the  classes  of  ware,  various  devices  being 
employed  for  supporting  the  ware  in  position  so  as  to  avoid 
distortion,  etc.,  and  glost  placing  differs  in  some  respects 
from  biscuit  placing,  for  in  the  latter  there  is  no  glaze  to 
melt  and  stick  together  pieces  which  may  be  in  actual 


CERAMIC  INDUSTRIES  209 

contact ;  in  glost  placing,  pieces  must  not  be  allowed  to 
touch  one  another,  and  to  prevent  this,  pointed  and  sharp - 
edged  supports  are  used,  so  as  to  leave  as  little  marking  as 
possible  when  (for  commercial  reasons)  a  number  of  pieces 
have  to  be  placed  in  the  same  saggar.  Saggars  are  piled 
up  in  "  bungs  "  or  columns,  and  these  bungs  are  arranged 
in  successive  "  rings  "  in  the  ovens.  The  products  of  com- 
bustion (including  flames)  pass  along  the  interspaces  between 
the  saggars. 

Effects  of  Firing. — The  effects  of  firing  ceramic  wares 
appear  in  regular  sequence  when  the  temperature  is  not 
raised  too  rapidly.     In  the  biscuit  firing,  water  (which  may 
contain  sulphuric  acid,  derived  from  sulphur  in  the  fuel) 
is  often  deposited  on  ware  below  100°  C.     Between  100° 
and  120°  C.  this  water  evaporates,  along  with  the  other 
water  present  in  the  ware  as  moisture.     From  200°  to  480°  C. 
organic  matter  is  burned  away,  water  is  driven  off  from 
hydrated  oxide  of  iron,  and  certain  hydrated  silicates  may 
possibly  also  lose  their  water.     In  the  next  stage,  between 
480°  and  600°  C.,  the  essential  substance  of  clay  is  decom- 
posed, and  loses  its  combined  water.     At  still  higher  tem- 
peratures, between  800°  and  900°  C.,   calcium  carbonate 
(when  present)  is  decomposed  into  lime  and  carbon  dioxide. 
Finally,  with  further  rise  of  temperature,  the  various  oxides 
recombine   to   form   double   silicates,  or   alumino-silicates, 
etc.,  from  900°  up  to  about  1300°  C.     In  the  case  of  hard 
porcelain,  sillimanite  is  formed  at  the  higher  temperatures. 
The  iron  speck  which  often  appears  on  ware  after  a  sharp 
firing  of  a  biscuit  oven,  probably  arises  from  reaction  between 
iron   compounds,   water  vapour,    and   organic  substances, 
when  the  firing  is  carried  on  too  quickly  between  400°  and 
500°  C.,  that  is,  when  the  whole  oven  is  just  getting  red. 
Before  contraction  commences,  there  is  generally  a  small 
though  distinct  expansion,   probably   due   mainly  to  the 
physical  action  of  the  escaping  gases  and  vapours.     In  the 
case    of   earthenware,    contraction   proceeds   very    rapidly 
between  900°  and  1300°  C.,  and  in  a  body  like  parian  con- 
taining  a   much   larger   proportion   of   alkali,    contraction 
c.  14 


210  SILICA   AND   THE  SILICATES 

takes  place  similarly  between  840°  and  1160°  C.  or  there- 
abouts. The  limits  are  similarly  modified  in  other  cases, 
according  to  the  proportion  of  alkali  present.  The  action 
of  the  alkali  is  modified  in  china  bodies  by  the  presence  of 
the  bone-ash,  and  though  china  undergoes  great  contraction, 
the  upper  limit  is  higher  than  for  parian  and  other  bodies 
high  in  alkali. 

Besides  the  contraction  and  changes  in  chemical  com- 
position of  bodies,  caused  by  biscuit  firing,  there  are  also 
usually  changes  in  colour,  hardness,  tenacity,  and  density. 
Hardness  is  always  associated  with  contraction,  and  in- 
creases along  with  the  latter. 

Colour  Changes  depend  on  the  amount  and  nature  of 
the  iron  compounds  present,  on  the  presence  or  absence  of 
certain  other  impurities  which  may  affect  colour,  and  on 
the  firing  atmosphere  (whether  oxidizing  or  reducing).  A 
creamy  tint  is  given  by  J  to  ij  per  cent,  of  iron  oxide,  as 
in  ball  clays  ;  2  to  4  per  cent,  of  iron  oxide  gives  shades  of 
yellow  (cane  or  buff)  ;  more  than  4  per  cent,  iron  oxide 
gives  salmon,  bright  red,  and  chocolate  tints,  terra-cotta 
containing  about  7  to  8  per  cent,  iron  oxide. 

Cla3^s  containing  iron  oxide  are  generally  yellow  or  red 
in  their  natural  condition,  and  the  colour  becomes  deeper 
after  firing.  Iron  sulphide  (pyrites)  and  iron  carbonate 
give  clays  a  dark-grey  colour,  inclining  to  bluish  or  greenish  ; 
these  tints  are  as  a  rule  completely  changed  after  firing, 
lyime  or  magnesia  usually  lightens  the  colour  of  a  fired  clay 
containing  iron,  owing  to  the  formation  of  nearly  colourless 
double  silicates.  This  is  well  exemplified  by  the  yellow 
London  malm  bricks,  made  of  chalk  and  a  red  brick  earth. 
Carbonaceous  matter  in  clay  also  tends  to  bleach  it,  because 
of  the  reduction  of  ferric  oxide  to  ferrous  oxide  during  the 
firing.  In  the  case  of  Staffordshire  blue  bricks,  a  ferruginous 
marl  is  fired  to  a  high  temperature,  and  the  fires  are  then 
damped  down  with  slack.  The  colour  of  the  bricks  is  due 
to  formation  of  magnetic  oxide  (Fe3O4). 

General  Course  of  Biscuit  Firing.— The  temperature 
should  be  raised  slowly  to  about  120°  C.  all  through.  Then 


CERAMIC  INDUSTRIES  211 

it  should  rise  more  quickly  to  about  450°  C.  (when  the  bags 
appear  just  red),  then  more  slowly  up  to  600°  C.  (the  oven 
being  then  a  solid  red  throughout)  ;  then  it  should  rise  very 
quickly  to  950°  C.,  and  finally  it  should  rise  steadily  to  the 
finishing  temperature,  followed  by  a  soak  according  to  the 
size  and  thickness  of  the  ware.  Mr.  Joseph  Burton  suggests, 
for  the  best  course  of  earthenware  biscuit  firing,  about 
15  hours  up  to  120°  C.,  20  hours  from  120°  to  450°  C., 
i6J  hours  from  450°  to  600°  C.,  16  hours  from  600°  to  950°  C., 
and  20  hours  from  950°  to  1140°  C.  (a  total  of  87 J  hours), 
followed  by,  say,  3  hours'  soak,  and  then  cooled  down  to 
660°  C.  in  1 6  hours.  In  some  works  the  total  firing  period 
from  starting  to  reaching  top  heat  is  20  to  30  hours  less  than 
this.  For  china,  the  biscuit  firing  can  be  taken  on  more 
rapidly  up  to  600°  C.  than  with  earthenware. 

For  the  control  of  the  firing,  thermo-electric  pyrometers 
or  electrical  resistance"  pyrometers  are  sometimes  used, 
with  or  without  recording  apparatus.  But  more  commonly 
other  contrivances  are  employed,  such  as  pyrometric  cones 
(or  Seger  cones,  which  soften  and  bend  over  or  collapse  at 
fairly  definite  temperatures),  Holdcroft's  bars  (which  bend 
under  their  own  weight  at  corresponding  temperatures), 
B  tiller's  rings  (which  indicate  temperature,  etc.,  by  the 
measured  contraction),  and  small  "  bits  "  made  of  certain 
other  clay  mixtures  (sometimes  the  regular  body  com- 
position used  at  the  factory),  the  contraction  of  which  is 
measured  by  means  of  a  gauge.  The  last-named  represents 
the  old  Wedgwood  pyrometer.  Within  recent  years  Messrs. 
Wengers,  L,td.,  have  introduced  trial  pieces  termed  "  calo- 
rites,"  which  are  short  rods  or  bars  somewhat  resembling 
Holdcroft's  thermoscope  bars,  but  only  one  end  is  supported 
in  a  socket,  the  other  end  standing  out  free  to  bend  as  the 
proper  firing  conditions  are  approached.  The  rings  and 
"  bits,"  depending  on  contraction,  have  to  be  drawn  from 
the  ovens  at  convenient  intervals  for  measurement,  but  the 
cones  and  bars  are  placed  so  as  to  give  their  indications, 
whenever  required,  by  simple  inspection  through  spy-holes. 
For  ascertaining  the  maximum  temperature  reached  in 


212  SILICA   AND   THE  SILICATES 

certain  parts  of  the  oven,  other  means  are  sometimes 
employed,  such  as  Watkin's  Heat  Recorders,  which  consist 
of  refractory  rods,  each  with  a  number  of  pellets  melting  at 
different  temperatures;  the  maximum  temperature  in  this 
case  is  the  melting  temperature  of  the  pellet  which  was  the 
last  to  show  distinct  indications  of  fusion. 

GRAZING  OF  EARTHENWARE,  ETC. 

Most  ceramic  bodies  are  more  or  less  porous  after  being 
fired,  and  in  order  to  render  the  surface  impervious  to  water 
and  other  liquids  a  glaze  (or  glassy  coating)  is  applied. 
This  glaze  when  properly  fired  on  the  ware  has  also  a  decora- 
tive effect,  for  the  sake  of  which  it  is  commonly  applied 
also  on  vitrified  bodies  which  are  themselves  practically 
impermeable  to  liquids.  The  essential  qualities  of  a  good 
glaze  are  :  (a)  suitability  to  the  body,  as  regards  both  com- 
position and  behaviour  when  heated  and  cooled,  that  it 
will  neither  craze  (form  very  fine  cracks)  nor  peel  (chip  off 
from  edges  of  ware)  ;  (b)  sufficient  hardness  to  resist  wear, 
especially  when  used  for  cooking  or  other  domestic  pur- 
poses ;  (c)  power  of  resisting  water  or  acid  vapours  and  (in 
many  cases)  all  acids  other  than  hydrofluoric  acid  ;  (d)  clear 
transparency  and  brilliance  following  prolonged  exposure 
to  gradually  increasing  temperature.  It  should  not  be  thick 
enough  to  obscure  the  modelling.  The  melted  glaze  should 
have  sufficient  tenacity  not  to  run  off  vertical  or  sloping 
surfaces,  but  should  not  be  too  stiff  to  flow  smooth  on  flat 
surfaces.  It  should  dissolve  colouring  materials  (metallic 
oxides)  without  forming  unsightly  separations.  It  should 
not  act  too  strongly  on  underglaze  colours.  The  same 
purposes  are  served  by  an  enamel,  which  differs  from  a 
glaze  in  being  opaque  and  not  so  completely  vitrified. 
Enamels  are,  in  fact,  often  nothing  more  than  lead  glazes 
containing  some  tin  oxide,  which  makes  them  opaque.  Both 
glazes  and  enamels  may  be  variously  coloured  by  the  addition 
of  certain  oxides  of  metals,  etc. 

These  stipulations  are  met  by  the  salt  glaze  on  stone- 
ware, and  the  glaze  on  hard  porcelain,  and — though  not 


CERAMIC  INDUSTRIES  213 

quite  so  completely — by  the  glaze  on  the  best  English  bone 
porcelain.  Most  other  glazes  regularly  used  are  more  or 
less  defective  in  the  respects  indicated,  though  they  may 
answer  for  most  purposes  which  a  glaze  is  intended  to  serve. 
Glaze  is  applied  to  pottery,  etc.,  in  most  cases  in  one 
of  the  following  ways  : — 

(1)  By    dipping    or  immersion.     This   is   the    usual 
method  for  fine  earthenware,  china,  etc.,  and  is  the  quickest 
and  cheapest  way  of  glazing  large  quantities  of  ware.     The 
finely  ground  materials  are  mixed  with  a  suitable  proportion 
of  water  (as  previously  ascertained  by  actual  trials,  and 
afterwards  checked  by  finding  the  weight  of  a  pint)  in  a 
dipping  tub  or  trough,  and  the  dipper  plunges  the  articles 
singly  into  the  glaze  for  a  very  short  time,  taking  precautions 
to  get  an  even  coating  of  glaze  on  the  ware.     The  excess 
of  glaze  is  allowed  to  run  off  while  the  ware  is  resting  on  a 
lawn  or  fine  netting.     The  porous  surface  of  the  biscuit 
ware  absorbs  water,  and  a  deposit  of  glaze  accumulates. 
This  causes  the  glaze  left  in  the  tub  to  get  gradually  heavier, 
and  water  has  to  be  added  from  time  to  time  to  restore  the 
original  strength   approximately,   which  may  be   checked 
either  by  weighing  a  pint  or  by  means  of  a  gauge  in  the  form 
of  a  simple  hydrometer.     Most  dippers  can  judge  by  the 
feel  of  the  glaze  on  putting  a  hand  in  it.     Adjustments 
have  also  to  be  made  for  marked  differences  in  porosity  of 
the  biscuit  ware  (due  to  differences  in  the  biscuit  firing), 
and  for  various  other  circumstances,  as  differences  in  thick- 
ness of  ware,  different  shapes,  etc.     Cleanliness  is  particularly 
necessary  throughout. 

(2)  By  sprinkling   (or  watering)   the  ware  with  glaze 
of  creamy  consistency.     This  method  is  resorted  to  in  the 
case  of  ware  which  is  vitreous  in  the  biscuit  state,  so  that  it 
has  practically  no  power  of  absorption.     The  glaze  is  allowed 
to  run  over  the  piece  while  it  is  moved  to  and  fro  until  it  is 
entirely  covered,  when  a  slight  jerk  causes  any  superfluous 
glaze  to  drop  off.     The  same  plan  is  adopted  for  glazes  of 
different  colour  used  on  the  same  piece,  the  outside  being 
glazed  by  dipping  and  the  inside  by  means  of  glaze  ladled  in. 


214  SILICA   AND  THE  SILICATES 

(3)  By  volatilization.  This  is  effected  by  first  raising 
the  biscuit  ware  to  a  suitable  temperature,  and  then  putting 
the  glazing  material  (preferably  damp)  in  the  oven  through 
special  openings  in  the  crown,  and  through  the  mouths. 
This  is  the  method  used  for  salt  glazing,  the  salt  volatilizing 
and  penetrating  to  the  ware  in  the  saggars  (which  are  not — 
like  those  in  which  dipped  ware  is  placed — practically  closed 
to  fumes  and  gases  by  wads,  that  is,  rolls  of  clay  carefully 
fixed  all  round  where  one  saggar  rests  on  another).  Mutual 
chemical  action  takes  place,  and,  it  is  often  stated,  with 
formation  first  of  soda  and  hydrochloric  acid  gas,  the  soda 
then  combining  with  silica  on  the  surface  of  the  body  to 
form  a  hard  and  insoluble  sodium  silicate.  As  has  been 
pointed  out  by  Dr.  J.  W.  Mellor,  among  others,  it  is  more 
probable  that  the  salt,  steam,  and  body  materials  react 
together  forming  a  double  silicate  of  soda  and  alumina  as  a 
coat  of  glaze  both  inside  and  outside.  The  salt  glaze  is  also 
liable  to  contain  small  amounts  of  other  bases,  especially 
lime.  The  salt-glazing  process  is  only  suitable  for  bodies 
containing  a  considerable  proportion  of  uncombined  silica. 
The  process  of  smearing  is  essentially  the  same,  but  in  this 
case  the  saggars  are  washed  with  the  glaze,  and  powdered 
glaze  in  cups  is  put  in  the  saggars.  The  glaze  at  a  certain 
heat  volatilizes,  and  the  ware  in  the  saggars  becomes  glazed. 
The  advantage  of  smearing  is  that  all  cavities  and  incisions, 
however  deep  or  fine  they  may  be,  are  glazed  without 
losing  any  sharpness  of  outline.  Smears  consist  of  two  or 
more  (rarely  only  one)  of  the  following :  red  lead  (or 
sometimes  litharge),  Cornish  stone,  salt,  potash,  nitre  (salt- 
petre), bone-ash,  flint,  lime. 

Flows  are  similar  to  smears,  and  were  introduced  (about 
1840)  with  the  object  of  making  the  cobalt  blue  and  some 
other  colours  spread  or  flow.  The  same  substances  are 
used  as  for  smears,  and  sometimes  whiting,  alum,  sal-am- 
moniac. The  flow  in  small  biscuit  cups  is  put  in  the  saggars 
with  the  glazed  ware,  and  the  fumes  produced  when  firing 
cause  certain  colours  to  flow  slightly  into  the  melted 
glaze. 


CERAMIC  INDUSTRIES  215 

(4)  By  dusting  the  ware  with  dry  powdered  glazing 
material  (as  red  lead  or  litharge).     This  method  is  generally 
used  in  the  clay  state,  while  yet  somewhat  moist,  and  the 
ware  undergoes  a  single  firing,  in  which  both  body  and  glaze 
are  matured  together.     Such  dry  dust  is  very  injurious  to 
the  health  of  those  who  have  to  use  it. 

(5)  Painting  with  a  brush,  employed  in  a  few  exceptional 
cases. 

(6)  Spraying,    Blowing,  or   Aerographing   is  rather 
a  decorative  process  than  mere  glazing,  colours  or  coloured 
glazes  being  chiefly  used. 

Glazes  fall  conveniently  into  four  classes,  according  to 
composition : — 

(1)  Alkaline  Glazes,  which  are  mainly  silicates  of  the 
alkalies  (potash  and  soda,  more  particularly  soda)  or  of  the 
alkalies  and  lime.     Examples  are  salt  glaze,  and  the  remark- 
able ancient  Egyptian  turquoise  glaze. 

(2)  Felspathic   Glazes    (consisting   of   silica,    alumina, 
alkaline  and  alkaline  earthy  bases),  which  contain  a  large 
proportion  of  felspar  or  felspathic  rock,  sometimes  softened 
to  a  certain  extent  by  admixture  of  more  fusible  materials. 
Examples  are  the  glaze  of  hard  porcelain  and  brick  glazes, 
as  well  as  some  glazes  of  English  porcelains,  borax  being 
added  in  the  latter  cases  to  increase  the  fusibility. 

(3)  Lead  Glazes,    consisting   mainly    of   silicates   and 
boro-silicates   of   alkalies   and  alkaline   earths,   with  some 
alumina,  and  softened  by  addition  of  lead  oxide  (introduced 
either  as  red  lead  or  white  lead,  the  latter  being  practically 
a  combination  of  carbonate  and  hydroxide  of  lead).     They 
often  contain  felspathic  materials.    Examples  are  all  ordinary 
transparent  glazes  for  English  earthenware,  and  those  for 
most  English  porcelains. 

(4)  Enamels  or  opaque  glazes,  which  generally  have  the 
composition  of  lead  glaze,  but  are  rendered  opaque  by  the 
addition  of  tin  oxide,  oxide  of  arsenic,  bone-ash,  or  zirconia, 
etc. 

I/ead  glazes  may  be  either  raw  (unfritted)  or  fritted. 

In  raw  glazes  the  components  are  merely  ground  together, 


2i6  SILICA   AND   THE  SILICATES 

the  harder  materials  being  put  on  the  mill  to  be  partly 
ground  first,  and  the  softer  materials  being  added  later. 

In  the  case  of  fritted  glazes,  all  the  substances  soluble  in 
water — including  soda  ash,  borax,  boracic  acid,  potash 
(pearl  ash),  nitre  (saltpetre),  etc. — with  some  of  the  others 
(as  whiting,  flint,  etc.),  are  first  fritted  together,  and  the  frit 
(or  imperfect  glass)  is  afterwards  ground  like  the  harder  com- 
ponents of  raw  glazes,  the  softer  ingredients  being  added  at  a 
late  stage.  Fritting  is  necessary  for  glazes  containing  much 
lime,  magnesia,  or  baryta  (which  are  not  themselves  fusible) 
in  order  to  promote  combination  which  might  otherwise 
need  a  high  temperature  to  start.  For  fritting  small  quan- 
tities of  glaze  materials,  clean  flinted  saggars  or  crucibles 
are  used,  and  for  large  quantities,  special  reverberatory 
furnaces  called  frit  kilns  are  employed. 

The  composition  of  glazes  varies  within  wide  limits 
according  to  the  duration,  maximum  temperature,  and  other 
conditions  of  firing.  Their  composition  is  conveniently 
represented  by  the  molecular  formulae  employed  by  Seger, 
which  show  the  relative  molecular  proportions  of  the  different 
oxides  present,  the  alumina  and  silica  being  kept  separate, 
and  the  basic  oxides — collectively  termed  the  fluxes  and 
represented  as  RO — being  taken  as  constituting  one  mole- 
cular proportion  jointly  ;  boron,  when  present,  is  calculated 
separately  as  B2O3. 

According  to  Seger,  the  glazes  in  actual  use  fall  within 
the  limits  indicated  by  the  glaze  formulae  for  the  respective 
classes.  Ordinary  earthenware  and  fine  French  faience, 
RO  :  i'5SiO2  to  RO  :  3SiO2.  English  and  German  white 
earthenware,  RO  :  o'oiAl2O3  :  2*5SiO2  to  RO  :  0'4A12O3 : 
4'5SiO2.  Hard  porcelain  glazes,  RO  :  0'5A12O3  :  5SiO2  to 
RO  :  I'25A12O3  :  i2'5SiO2. 

The  SiO2  often  includes  boric  acid  in  proportions  from 
0*25  to  0*05  of  the  silica.  RO=  potash,  soda,  lime,  magnesia, 
baryta,  lead,  and  the  colouring  oxides.  The  corresponding 
firing  temperatures  range  from  below  the  melting  point  of 
silver  (cone  oio)  to  cone  18. 

Glazes  containing  no  alumina  tend  to  become  devitrified, 


CERAMIC  INDUSTRIES  217 

and  are  also  liable  to  run  off  or  soak  into  the  porous  body. 
Alumina  in  glazes  removes  these  risks,  and  at  the  same 
time  raises  the  melting  point.  The  composition  of  ordi- 
nary glasses  can  be  represented  by  Na2O.CaO  :  5SiO2,  or 
RO  :  2'5SiO2,  which  comes  within  the  first  pair  of  the  above 
limits ;  a  larger  proportion  of  silica  causes  the  glass  to  be 
easily  devitrified,  whereas  a  lower  proportion  of  silica 
causes  loss  of  ductility  and  viscosity  ;  in  each  case  it  becomes 
worse  as  regards  working  qualities.  Devitrification  consists 
of  a  separation  of  silicic  acid,  or  of  silicates,  in  crystalline 
forms  which  cloud  the  glass  and  destroy  the  surface  lustre, 
producing  a  mass  resembling  porcelain  if  heated  for  a  long 
time  at  softening  temperature.  Such  separations  can  only 
be  removed  by  heating  the  glass  up  to  a  very  liquid  con- 
dition. 

The  silicates  of  magnesia,  lime,  baryta,  and  strontia 
alone  are  stony  or  slag-like  in  appearance,  and  therefore  do 
not  form  transparent  glasses. 

For  the  same  acidity,  it  is  found  in  practice  that  the 
fluidity  increases  with  the  number  of  fluxes  (basic  oxides) 
present. 

The  leadless  glazes  mostly  contain  lime  and  alkali 
(potash  or  soda,  or  both).  Sometimes  magnesia,  baryta,  or 
zinc  oxide  is  used,  or  more  than  one  of  these. 

The  alkali-lime  glazes  have  a  much  more  limited  appli- 
cation than  lead  glazes,  and  much  more  care  is  necessary 
in  order  to  get  good  results  from  them.  The  permissible 
proportions  range  between  0'2  equivalent  of  potash  or  soda 
to  0'8  equivalent  of  lime,  and  0*6  equivalent  of  potash  or 
soda  to  0*4  equivalent  of  lime.  As  regards  the  alkalies  it 
appears  to  be  of  no  consequence  whether  potash  or  soda 
be  used,  except  that  soda  gives  a  slightly  more  fusible  glaze 
than  potash,  and  both  present  together  make  it  still  more 
fusible.  Glazes  high  in  alkali  are  better  suited  for  bodies 
rich  in  quartz,  glazes  low  in  alkali  are  better  suited  for  a 
high  alumina  body.  A  great  peculiarity  of  lime  glazes  is 
that  they  require  a  high  content  of  alumina  to  produce 
transparent  glasses  at  all  on  slow  cooling,  as  otherwise  they 


2i8  SILICA   AND   THE  SILICATES 

always  give  dim  and  milky  glasses — excepting  glazes  corre- 
sponding to  ordinary  glasses,  and  these  would  craze  badly 
on  bodies  high  in  alumina.  A  silica  content  of  4  equivalents 
is  needed  to  give  a  sufficient  gloss,  and  at  least  0*5  equivalent 
of  alumina  is  necessary.  With  lead  glazes  the  alumina 
seems  to  tend  always  to  make  glasses  or  glazes  more  dim 
and  milky,  and  in  the  lead  glazes  used  the  alumina  is  much 
less  than  the  minimum  amount  necessary  for  the  lime 
glazes.  More  than  one  equivalent  of  boric  acid  in  these 
lime  glazes  tends  to  produce  a  milky  appearance,  but 
at  least  0*5  equivalent  is  necessary  to  give  sufficient 
fusibility.  Hence  with  lime-alumina  glazes  to  mature  at  a 
temperature  not  exceeding  the  melting  point  of  gold,  the 
frits  to  be  used  (with  addition  of  10  per  cent,  kaolin)  must 
be  kept  within  the  limits  0'6K2O(Na2O).0'4CaO  :  o*5A!2O3  : 
4SiO2.o*5B2O3,  and  o'2K2O(Na2O).o-8CaO  :  0'6A12O3  : 
o-5SiO2.iB2O3. 

Such  formulae  enable  the  percentage  composition  of  a 
glaze  or  frit,  etc.,  to  be  calculated,  and  conversely  the 
formula  can  be  obtained  from  the  percentage  composition. 
With  fritted  glazes,  allowance  has  to  be  made  for  the  loss 
of  weight  (CO2,  H2O,  etc.)  sustained  on  fritting. 

Working  out  a  suitable  glaze  for  a  particular  body 
would  be  difficult  and  tedious  by  haphazard  trials,  but  by 
starting  with  a  mixture  based  on  a  definite  chemical  formula 
within  known  limits,  an  experienced  investigator,  even 
without  much  knowledge  of  practical  potting,  need  not 
take  long  in  reaching  the  desired  result. 

Similar  calculations  may  be  made  in  the  case  of  body 
compositions,  but  with  them  it  is  usually  preferred  to 
take  the  alumina  (instead  of  the  fluxes  jointly)  as  i 
molecule. 

Some  approximate  values — accurate  enough  for  all 
ordinary  calculations — of  combining  weights  for  certain 
ceramic  materials  are  appended : — 


CERAMIC   INDUSTRIES 


219 


Substance. 
Alumina 
Barium  oxide  (Baryta) 

,,       carbonate 

sulphate 
Borax 

„  anhydrous  (Borax  glass) 
Boric  acid 

,,     oxide 
Calcium  oxide  (L,ime) 

„       carbonate     . . 

,,       sulphate  (Gypsum) . . 
Ferric  oxide 
Lead  oxide  (Litharge) 
Red  lead  (Minium) 
White  lead      .. 
Magnesia 
Magnesite 

Potash  (Potassium  oxide)     . . 
Potassium  carbonate   (Pearl- 
ash) 

Nitre  (Saltpetre) 
Potash  Felspar  (Orthoclase) 
Silica,  (Flint,  Quartz,  etc.)    . . 
Soda  (Sodium  oxide) 
Soda  ash  (Sodium  carbonate) 
Soda  crystals 
Sodium  chloride  (Salt) 
Sodium  nitrate  (Cubic  nitre) 
Tin  oxide 

Titanium  oxide  (Rutile) 
Zinc  oxide 

Kaolinite    and  Clayite    (Clay 
substance) 


Molecular 
or  combining 

Formula.  weight. 

A12O3  102 

BaO  153 

BaCO3  197 

BaSO4  233 

Na2B4O7.ioH2O  382 

Na2B4O7  202 

H3BO3  62 

B2O3  70 

CaO  56 

CaCO3  100 

CaSO4.2H2O  172 

Fe2O3  160 

PbO  222 

Pb304  683 

2PbC03.Pb(OH)2  773 

MgO  40 

MgC03  84 

K20  94 

K2CO3.2H2O  154 

KNO3  101 

K2O.Al203.6Si02  559 

SiO2  60 

Na2O  62 

Na2CO3  106 

Na2CO3.ioH2O  286 

NaCl  58*5 

NaNO3  85 

SnO2  150 

TiO2  80 

ZnO  81 

Al2O3.2SiO2.2H2O  259 


It  should  be  borne  in  mind  that  any  given  formula  or 
recipe  for  a  glaze  is  practically  useless  in  itself,  inasmuch  as 


220  SILICA   AND   THE  SILICATES 

it  is  suitable  only  for  a  particular  body  or  bodies,  and  even 
with  them  only  under  fairly  definite  conditions  of  firing. 
Such  a  formula  or  recipe,  however,  may  serve  as  a  con- 
venient starting  point,  providing  the  investigator  has 
the  requisite  knowledge  and  experience  to  make  good  use 
of  it. 

Seger  deduced  the  following  general  conclusions  as 
regards  the  chemical  and  physical  relations  between  the 
composition  of  the  body  and  that  of  the  glaze.  By  in- 
creasing the  clay  bonding  material  in  a  body,  its  expansion 
with  a  given  rise  of  temperature  is  decreased — and  crazing 
increases  with  the  same  glaze — but  addition  of  a  plastic 
clay  decreases  the  coefficient  of  expansion  in  a  less  degree 
than  the  less  plastic  kaolin  or  China  clay.  Increase  of 
felspar  also  reduces  the  contraction  (or  expansion)  of  the 
body.  Increase  of  quartz  (free  silica)  increases  the  co- 
efficient of  expansion — and  peeling  increases  with  the  same 
glaze — and  this  increase  is  more  rapid  with  increasing 
fineness  of  grain.  The  coefficient  of  expansion  is  also  in- 
creased by  increasing  hardness  of  the  biscuit  firing,  so  that 
very  easy  biscuit  firing  promotes  crazing,  whilst  very  hard 
biscuit  firing  promotes  peeling  and  cracking  of  the  edges. 
Seger  gives  the  following  as  practical  means  of  adjusting 
defects  arising  from  differences  in  contraction  of  body  and 
glaze  on  cooling  : — 

I.  To  remedy  crazing,     (i)  Reduce  the  plastic  bonding 
material  (clay)  in  the  body,  and  at  the  same  time  increase 
the  silica.     (2)  Replace  part  of  the  clay  substance  used  as 
kaolin  by  an  equivalent  quantity  in  the  form  of  plastic  clays 
(such  as  ball  clay).     (3)    Reduce  the  felspar   (or  Cornish 
stone).     (4)  Use  quartz  (free  silica)  in  a  more  finely  ground 
condition.     (5)  Fire  harder  for  the  biscuit. 

II.  To  remedy  peeling,   or  cracking   of  the  edges,   or 
breaking  away  of  attached  parts  such  as  handles,  spouts, 
etc.     (i)   Increase  the  plastic  clay  substance,   and  at  the 
same  time  decrease  the  quartz  (free  silica).     (2)  Replace  part 
of  the  clay  substance  used  as  plastic  clay  by  an  equivalent 
quantity   as  kaolin.     (3)   Increase  the  felspar   (or  Cornish 


CERAMIC  INDUSTRIES  221 

stone).  (4)  Use  quartz  (free  silica)  less  finely  ground. 
(5)  Fire  easier  for  trie  biscuit. 

Seger  also  points  out  that  the  coefficient  of  expansion 
of  the  glaze  decreases  with  increase  of  silica,  and  increases 
with  increase  of  fluxes  (bases).  The  tendency  to  crazing 
decreases — and  tendency  to  peeling  increases — by  increasing 
the  content  of  boric  acid  to  replace  part  of  the  silica.  Hence 
boric  acid  decreases  the  coefficient  of  expansion  of  a  glaze. 
The  relative  fusibility  of  lead-soda  glaze,  lead-potash  glaze, 
barium-soda  glaze,  lime-soda  glaze,  and  lead-magnesia 
glaze,  is  in  the  order  named,  the  lead-soda  glaze  being  the 
most  fusible.  The  tendency  to  crazing  is  least  in  the  mag- 
nesia-soda glaze,  and  greatest  in  the  lead-soda  glaze,  the 
others  being  intermediate  as  regards  fineness  of  the  network 
of  cracks.  In  case  peeling  should  take  place,  it  would  be 
most  pronounced  with  the  magnesia-soda  glaze,  and  decrease 
in  the  order  indicated  above.  If  alumina  to  the  extent  of 
o'iAl2O3,  0'2A12O3,  0'3A12O3,  be  introduced  into  a  glaze  of 
the  composition  o*5PbO,  o'5Na2O  :  2'5$iO2  (which  repre- 
sents a  normal  glass),  the  melting  point  rises  rapidly,  and 
at  the  same  time  the  product  becomes  clouded  and  enamel- 
like.  The  added  alumina  appears  to  exert  no  perceptible 
influence  on  the  expansion  or  contraction  of  glazes  with 
temperature  changes. 

Seger  gives  the  following  as  practical  means  to  remedy 
crazing  by  alteration  of  the  glaze,  (i)  Increase  of  the  silica 
content,  and  decrease  of  the  fluxes,  but  the  ratio  of  silica 
to  fluxes  must  not  exceed  that  of  a  trisilicate.  Further 
increase  of  silica  would  involve  addition  of  more  alumina 
as  well — and  also  rise  of  the  melting  point — or  otherwise 
a  tendency  towards  devitrification  would  be  developed. 
(2)  Increase  of  boric  acid  at  the  expense  of  the  silica,  the 
melting  point  of  the  glaze  being  lowered  thereby.  (3)  Sub- 
stitution of  a  fluxing  oxide  of  high  combining  weight  by  one 
of  lower  combining  weight,  thereby  raising  the  percentage 
of  silica,  and  also  raising  the  melting  point  of  the  glaze. 

Increase  in  thickness  of  the  glaze  layer  increases  the 
tendency  to  crazing.  A  glaze  which  crazes  after  a  short 


222  SILICA   AND  THE  SILICATES 

or  easier  fire  may  adhere  to  the  body  without  crazing  after 
a  long  and  hard  glost  firing.  This  is  explained  by  mutual 
chemical  action,  the  glaze  becoming  richer  in  alumina  by 
taking  up  some  from  the  body. 

Seger  gives  the  following  as  practical  means  to  remedy 
peeling,  cracking  of  the  edges,  etc.,  by  altering  the  glaze  : 

(1)  Lowering  the  silica  content  and  at  the  same  time  in- 
creasing the  fluxes,  but  the  ratio  of  silica  to  fluxes  must 
not  fall  below  that  of  a  bisilicate.     Below  this  ratio  silica 
can  only  be  reduced  by  reducing  alumina  at  the  same  time, 
which  would  also  have  the  effect  of  lowering  the  melting 
point  of  the  glaze.     (2)  Increase  of  silica  at  the  expense  of 
boric  acid,  which  without  other  changes  would  raise  the 
melting  point.     (3)  Substitution  of  a  fluxing  oxide  of  high 
combining  weight  (as  lead  oxide  or  barium  oxide)  for  one 
of  low  combining  weight  (as  lime  or  soda),  which  lowers 
the  percentage  of  silica  and  also  lowers  the  melting  point  of 
the  glaze. 

Seger's  rules  for  increasing  the  fusibility  of  a  glaze  are 
the  following  :  (i)  Decrease  the  silica  content,  and  thus 
raise  the  percentage  of  basic  fluxes,  keeping  within  the 
limiting  ratios  of  trisilicate  and  bisilicate  in  order  to  avoid 
devitrification  or  a  tendency  to  run  off  the  ware  respectively. 

(2)  Substitute  a  more  energetic  flux  for  a  less  energetic  one, 
but  retain  the  same  acidity.    (3)  Increase  the  number  of  fluxes 
present,  keeping  the  same  acidity.    (4)  Decrease  both  alumina 
and  silica,  keeping  within  the  limits  of  acidity  mentioned 
in  (i)  above.     (5)  Increase  the  boric  acid,  with  or  without 
corresponding  reduction  of  the  silica.     In  (i)  the  same  effect 
would   obviously   result   from  increase   of   fluxing   oxides, 
especially  of  lead  oxide.     The  brilliance  of  the  glaze  also 
increases   with   the    fusibility.     When   increased   fusibility 
is  produced  by  boric  acid,  the  resulting  glaze  is  also  more 
brilliant,  but  has  less  hardness  (being  more  easily  scratched), 
and  also  has  much  greater  action  on  underglaze  colours. 

All  these  general  statements  of  Seger  apply  strictly  for 
a  particular  range  of  conditions  as  regards  composition, 
firing,  etc.  Nevertheless  they  serve  as  useful  guides,  if 


CERAMIC  INDUSTRIES  223 

allowance  be  made  as  far  as  possible  for  any  important 
differences. 

Coloured  Glazes  are  prepared  by  the  addition  (in  suitable 
proportions)  of  certain  mineral  substances — chiefly  metallic 
oxides — to  a  white  glaze.  Thus  Salvetat  gives  a  white 
glaze  of  wide  adaptability  consisting  of  4  parts  red  lead, 
2  parts  flint,  and  i  part  calcium  borate  ;  this  gives  with 
addition  of  2  or  3  per  cent,  of  cobalt  oxide  a  blue  glaze, 
with  8  or  9  per  cent,  of  copper  oxide  a  bluish  green  glaze, 
with  4  per  cent,  of  ferric  oxide  a  yellow  glaze,  or  with  3  per 
cent,  manganese  oxide  a  brown  or  purplish-brown  glaze. 
Different  shades  would  be  obtained  by  increasing  or  de- 
creasing the  amount  of  colouring  oxide  added.  Similar 
proportions  might  be  tried  with  other  glazes,  but  it  should 
always  be  borne  in  mind  that  different  glazes  may  react 
differently  on  colouring  oxides.  Many  of  the  colours 
applied  on  the  glaze  are  not  available  for  underglaze  decora- 
tion owing  to  the  destructive  action  of  the  melted  glaze. 
All  the  colouring  metallic  oxides,  with  the  single  exception 
of  copper  oxide,  form  compounds  with  a  lower  expansion 
than  that  of  lead  oxide.  That  is  why  it  is  so  difficult  to 
obtain  satisfactorily  certain  colours  (especially  turquoise) 
from  copper. 

Crystalline  Glazes  are  produced  as  the  result  of  the 
devitrification  of  the  glassy  composition.  The  crystals 
most  readily  produced  are  those  of  zinc  silicate,  which  can 
be  obtained  of  a  size  easily  visible  to  the  naked  eye,  by  the 
slow  cooling  of  a  leadless  glaze  containing  zinc  oxide  and 
potash.  They  tend  to  form  in  rosettes  distributed 
irregularly.  The  crystals  can  be  coloured  by  adding  nickel 
oxide  or  cobalt  oxide  to  the  glaze.  The  author  has  himself 
obtained  in  this  way  fine  blue  crystals  by  addition  of  com- 
mercial oxide  of  nickel.  Titanium  oxide  also  favours  crystal- 
lization, but  the  crystals  are  minute.  Manganese  oxide 
may  likewise  give  crystallization. 

The  drying,  placing,  and  firing  of  glazed  ware,  are 
carried  out  in  much  the  same  way  as  the  corresponding 
operations  for  unglazed  ware,  with  the  important  difference 


224  SILICA   AND  THE  SILICATES 

that  special  precautions  have  to  be  taken  that  the  pieces 
shall  not  touch  each  other,  for  otherwise  they  would  become 
stuck  together  at  points  of  contact  by  the  melting  and  sub- 
sequent solidification  of  the  glaze.  In  the  biscuit  firing 
this  is  not  necessary,  except  in  the  case  of  vitrified  wares 
like  porcelain,  parian,  or  stoneware,  though  some  care  is 
needed  to  prevent  deformation  of  such  articles  as  dishes 
(open  and  covered),  cups,  and  similar  pieces  ;  in  most  cases 
smaller  pieces  may  be  put  inside  the  larger  hollow- ware. 
Another  important  difference  is  that  for  the  glost  firing 
wads  (small  rolls  of  clay)  are  laid  round  the  upper  edge  of 
each  saggar,  so  that  when  covered  the  saggars  become 
practically  dust-tight  and  air-tight,  or  at  any  rate  sufficiently 
so  to  prevent  smoke,  etc.,  from  entering  them.  As  there  is 
little  water  to  be  driven  off,  and  the  clay  body  has  been  pre- 
viously fired,  the  early  stages  of  gradual  heating  are  much 
shorter  in  glost  than  in  biscuit  firing,  with  the  result  that 
the  whole  firing  lasts  usually,  in  the  case  of  large  ovens, 
20  to  25  hours,  as  compared  with  45  to  55  hours  commonly 
allowed  for  firing  a  large  biscuit  oven. 

DECORATION. 

Decoration  may  be  under  the  glaze  or  over  the  glaze 
(on-glaze),  and  may  be  applied  by  painting  or  printing,  etc. 
Excluding  decoration  in  the  clay  state,  which  has  already 
been  alluded  to,  pottery  decoration  may  be  roughly  classed 
as  underglaze  decoration,  including  painting  and  printing, 
and  enamel  or  on-glaze  decoration,  including  painting, 
printing,  lithography,  gilding  and  silvering,  lustres. 

Some  eight  or  nine  years  ago,  the  author  conceived  the 
idea  that  bright-red  crystals  (in  outline  at  least)  might 
possibly  be  obtained  by  the  use  of  copper  compounds. 
With  this  object  in  view,  an  aqueous  solution  of  a  copper 
salt  (sulphate,  nitrate,  or  chloride)  was  applied  to  the  white 
glazed  surface  of  a  small  earthenware  vase,  and  when  crystals 
had  been  deposited  over  the  surface,  the  vase  was  dried, 
and  afterwards  fired  under  suitable  reducing  conditions. 
Two  trials  only  were  made,  and  red  crystal  forms  were 


CERAMIC  INDUSTRIES  225 

actually  obtained,  but  unfortunately  they  were  so  very 
small  and  so  crowded  as  to  possess  only  slight  decorative 
value.  The  trials  were  abandoned  for  a  time,  but  no 
favourable  opportunity  has  since  occurred  for  further 
testing  the  idea.  In  passing,  it  may  be  mentioned  that  the 
author  had  previously  discovered  that,  with  proper  firing, 
a  good  plain  copper-red  colour — giving  the  effect  of  a  fine 
red  glaze — can  be  readily  obtained  by  similar  application 
of  a  solution  of  a  copper  salt  to  the  carefully  cleaned  surface 
of  a  white  glazed  p'iece,  then  drying  and  firing  as  in  the  case 
just  referred  to.  It  was  further  ascertained  that  very 
satisfactory  liquid  for  this  purpose  is  a  solution  of  copper 
chloride  or  copper  nitrate  in  alcohol  (ordinary  methylated 
spirit  answering  very  well  instead  of  alcohol).  One  advan- 
tage of  this  method  is  that  little  or  no  cleaning  (or  polishing) 
is  necessary  after  the  firing.  It  may  be  added  that  the 
quality  of  the  colour  apparently  does  not  depend  on  strength 
of  solution,  for  solutions  of  several  different  strengths  were 
used,  and  comparatively  weak  solutions  gave  as  good  a 
colour  as  the  stronger  solutions.  It  cannot  be  too  strongly 
emphasized  that  the  conducting  of  the  firing  is  the  decisive 
factor  in  these  copper  reds. 

Before  leaving  the  subject  of  crystalline  glazes,  it  may  be 
pointed  out  that  the  foregoing  experiences  suggest  a  general 
method  of  perhaps  wide  application  for  obtaining  various 
crystalline  glazes,  coloured  or  uncoloured,  and  in  most  cases 
without  necessitating  reducing  firing  conditions.  For  in- 
stance, it  seems  possible  with  proper  precautions  to  get 
coloured  crystals  containing  zinc  by  the  use  of  a  solution 
of  zinc  sulphate  containing  some  isomorphous  nickel  sulphate 
or  cobalt  sulphate,  etc.  Such  crystals,  of  course,  would  not 
be  zinc  silicate,  and  perhaps  might  not  take  up  the  colouring 
material  easily.  It  would  seem  possible  that  the  application 
over  the  crystals  of  rather  dilute  silicate  of  soda  solution 
before  firing  might  result  in  the  formation  of  silicate,  though 
on  the  other  hand  too  high  a  temperature  might  be  necessary 
to  effect  that  change.  The  author  regrets  he  has  not  been 
able  to  explore  this  extensive  field  himself,  and  as  he  sees 
c-  15 


226  SILICA   AND  THE  SILICATES 

no  early  prospect  of  doing  so  tie  has  decided,  after  careful 
consideration,  to  give  others  the  chance  of  taking  up  the 
quest. 

The  very  earliest  decoration  consisted  in  ornamentation 
of  the  clay  by  scratched  lines  (sgraffito),  etc.  The  next 
decorative  device  (at  any  rate  in  this  country)  was  the 
application  of  white  and  coloured  slips  to  the  surface  of  the 
clay  before  glazing  ;  barbotine  or  slip  painting  is  a  modern 
style  of  decoration  of  the  same  character.  This  naturally 
led  to  painting  with  colours  when  the  common  clay  bodies, 
often  dark-coloured,  were  replaced  by  materials  which, 
after  firing,  yielded  a  lighter  ground  of  more  or  less  yellowish 
tint  (cream-coloured),  and  finally  white.  The  immediate 
object  of  painting  was  imitation  of  the  decoration  of  Chinese 
porcelain.  Printing  on  pottery,  etc.,  is  of  purely  English 
origin,  dating  from  early  in  the  second  half  of  the  eighteenth 
century.  Lithography  is  a  comparatively  recent  develop- 
ment. 

•  Printing  on  pottery  is  generally  believed  to  have  been 
introduced  by  John  Sadler,  at  Liverpool,  in  or  before  1756, 
that  year  being  the  first  which  has  been  verified  with  cer- 
tainty ;  it  was  used  on  Bow  porcelain  in  the  same  year, 
and  in  both  places  was  on  the  glaze.  Sadler  took  Guy 
Green  into  partnership.  Printed  enamels  (on  copper)  are 
known  to  have  been  made  at  Battersea  three  years  earlier. 
Underglaze  printing  is  first  known  at  Worcester  (on  porcelain) 
in  1757.  The  familiar  willow  pattern  was  first  produced  by 
John  Turner  in  1780  at  Caughley,  close  to  the  present 
Coalport.  About  1783  James  Robinson  and  Thomas  Lucas 
introduced  the  use  of  "  oils  "  and  the  method  of  washing 
the  transfer  paper  off  the  biscuit  ware,  and  the  process  of 
hardening  on  the  colours  before  glazing.  I^ater  improve- 
ments have  been  chiefly  in  matters  of  detail,  as  regards 
colours,  paper,  presses,  etc. 

The  design  is  engraved  on  a  copper  plate  (often  in  the 
form  of  a  cylinder  or  roller),  and  the  colour,  mixed  with  a 
special  oily  composition  known  as  printers'  oil — which 
enables  the  colour  to  adhere  to  the  biscuit  ware— is  spread 


CERAMIC  INDUSTRIES  227 

over  the  plate  and  well  rubbed  into  the  lines  and  hollows. 
Excess  of  colour  is  then  removed  by  means  of  a  large  flexible 
knife,  and  the  surface  of  the  plate  is  cleaned.  A  sheet  of 
special  paper  is  saturated  with  a  size  consisting  of  water  and 
soft  soap,  and  is  then  spread  over  the  engraved  plate.  By 
working  the  press,  which  is  only  a  modification  of  an  ordinary 
printing  press,  the  pattern  is  fixed  on  the  paper.  The  paper 
is  next  detached  from  the  plate,  the  waste  paper  cut  from 
it,  and  the  pattern  is  placed  on  the  ware  and  rubbed  well 
with  a  roll  of  flannel,  when  the  colour  is  absorbed  by  the 
porous  surface  of  the  biscuit  ware.  The  paper  is  easily 
washed  off  after  soaking  in  water,  and  the  ware  is  then  dried. 
The  oily  matter  is  in  most  cases  removed  by  a  special  firing 
(termed  "  hardening  on  ")  in  a  muffle  kiln,  at  a  comparatively 
low  temperature.  After  this  the  ware  is  ready  for  glazing. 

Enamel  Printing  is  done  in  the  same  manner  as  under- 
glaze  printing,  but  usually  with  the  special  enamel  colours. 
It  is  often  employed  to  print  outlines  which  are  then  filled 
in  by  painting.  It  is  also  much  used  for  badges,  lettering,  etc. 

The  chief  mineral  colouring  oxides  used  for  decoration 
in  ceramics  include  cobalt  oxide  (giving  blue  and  black), 
nickel  oxide  (green,  etc.),  copper  oxide  (green,  red,  brown, 
and  blue),  chromium  oxide  (green),  manganese  oxide  (brown, 
violet,  and  black),  iron  oxide  (red,  brown,  yellow,  green, 
black),  titanium  oxide  (yellow),  antimony  oxide  (yellow  and 
orange),  uranium  oxide  (yellow,  orange,  or  black),  iridium 
oxide  (black),  tin  oxide  (opaque  white),  besides  certain 
substances  such  as  chromate  of  iron  and  other  chromates, 
silicates,  etc.  The  so-called  native  chromate  of  iron,  con- 
sisting essentially  of  ferrous  oxide  and  chromic  oxide,  is 
not  a  true  chromate,  but  in  practice  acts  like  the  artificial 
chromate  obtained  by  precipitation ;  it  gives  browns.  A 
few  oxides  such  as  alumina,  zinc  oxide,  tin  oxide,  lime, 
which  alone  do  not  give  rise  to  characteristic  tints,  power- 
fully modify  the  colouring  effects  of  other  substances.  A 
notable  example  is  the  pink  or  crimson,  produced  by  tin 
oxide  and  lime  with  a  small  proportion  of  chromium  oxide  ; 
the  olive  green  produced  by  iron  in  presence  of  lime  is  another 


228  SILICA   AND   THE  SILICATES 

instance.  Blackis  produced  by  various  combinations  of  oxides, 
especially  the  oxides  of  iron  and  chromium  (or  the  equivalent 
chromate  of  iron),  manganese,  and  cobalt.  One  manu- 
facturer at  least  was  in  the  habit  of  obtaining  black  by 
simply  mixing  samples  of  various  colours.  Various  sub- 
stitutes for  tin  oxide  have  been  tried  to  get  opaque  white, 
but  the  only  one  to  approach  it  is  zirconia  or  preparations 
based  on  it,  such  as  the  "  terrar  "  used  for  that  purpose  in 
Germany.  Several  comparatively  rare  oxides,  as  those  of 
tungsten  and  vanadium,  have  been  used  for  the  preparation 
of  ceramic  colours  (yellow). 

It  may  also  be  noted  that  cobalt  oxide  alone  always  gives 
a  strong  blue  either  to  a  body  or  glaze,  inclining  to  violet 
in  the  latter  case  ;  mixed  with  oxide  of  zinc  it  gives  an 
ultramarine  blue,  and  mixed  with  alumina  it  gives  a  fine 
sky-blue  colour,  whilst  any  other  shade  of  blue  can  be 
obtained  by  mixing  with  other  colouring  oxides.  In  like 
manner  the  effect  of  manganese  oxide  depends  on  the  nature 
of  the  glaze  ;  alkaline  glazes  are  coloured  violet,  boracic 
glazes  brown,  and  lead  glazes  intermediate  tints.  So,  also, 
with  copper  oxide,  in  boracic  or  lead  glazes  it  gives  a  green 
colour,  but  in  alkaline  glazes  a  turquoise  blue ;  under 
reducing  conditions  alkaline  glazes  are  coloured  a  fine 
purplish  red,  but  boracic  or  lead  glazes  are  more  inclined  to 
orange  or  brown. 

Underglaze  Colours. — These  are  derived  from  a  limited 
number  o±  mineral  products,  which  are  capable  of  resisting 
the  action  of  the  melted  glaze,  and  consist  mainly  of  oxides 
of  cobalt,  chromium,  iron,  manganese,  and  antimony,  as 
well  as  certain  compounds  such  as  chromate  of  iron. 

To  assist  in  protecting  the  underglaze  colours  from  the 
action  of  the  glaze,  certain  other  materials  are  also  mixed 
with  the  colouring  substances,  such  as  flint,  China  clay,  etc. 
The  main  points  to  attend  to  are  to  get  the  materials 
thoroughly  well  mixed  together,  and  after  calcining  the 
mixture  (and  washing  when  necessary)  the  preparation 
should  be  ground  very  finely  to  get  the  best  results.  As 
pointed  out  by  A.  Granger,  intimate  admixture  can  be 


CERAMIC  INDUSTRIES  229 

easily  secured  by  using  soluble  substances,  preferably  salts 
which  when  heated  yield  the  colouring  oxides.  The  solutions 
are  mixed  in  proper  proportions,  and  then  evaporated  to 
drive  off  the  water.  An  alternative  method  is  suggested  by 
the  usual  way  of  preparing  purple  of  Cassius,  of  which  gold 
and  tin  are  essential  components  ;  two  or  more  oxides  may 
be  precipitated  together  from  a  mixed  solution,  and  the 
precipitate  is  washed,  etc.,  as  may  be  necessary.  This  plan 
is  followed,  with  good  results,  in  at  least  one  well-known 
British  factory. 

Enamel  or  On-glaze  Colours. — These  are  in  reality 
nothing  mere  than  coloured  glasses,  fusible  at  a  comparatively 
low  temperature,  which  at  a  red  heat  become  fixed  on  the 
glaze.  As  the  action  on  them  at  that  temperature  is  only 
slight,  a  much  greater  variety  is  available.  These  colours 
consist  of  the  colouring  substance  mixed  with  a  suitable 
flux.  All  are  fired  in  muffle  kilns,  and  are  classed  as  regular 
kiln  colours  and  hard  kiln  colours,  according  to  the  lower  or 
higher  proportion  of  flux  they  contain.  Nearly  all  the 
enamel  colours  are  prepared  by  grinding  the  components, 
then  melting  the  ground  mixture,  and  grinding  very  finely 
the  fused  product.  The  only  exceptions  are  the  colours 
prepared  from  gold  (largely  in  the  form  of  purple  of  Cassius), 
and  the  iron-red.  The  gold  colours  are  prepared  by  well 
mixing  the  components,  and  gently  calcining  the  mixture 
below  a  red  heat ;  the  colour  produced  in  the  kiln  ranges 
from  rose  through  carmine  and  marone  to  purple,  at  tempera- 
tures between  a  clear  red  heat  and  almost  white  heat.  The 
red  from  iron  contains  red  ferric  oxide,  obtained  by  careful 
calcination  of  dried  copperas  (ferrous  sulphate)  at  tempera- 
tures of  400°  to  420°  C.,the  colours  obtained  outside  this 
range  being  very  inferior  ;  a  suitable  flux  consists  of  6  parts 
lead  oxide,  2  parts  flint  (or  other  form  of  silica),  and  I  part 
borax  glass  ;  3  parts  of  this  flux  may  be  mixed  with  i  part 
of  red  oxide.  Some  colours,  as  the  uranium  black  and  the 
copper  red,  can  only  be  produced  under  reducing  conditions. 
From  the  author's  experience  it  may  be  stated  that  the 
production  of  fine  reds  from  copper  depends  much  more  on 


230  SILICA   AND  THE  SILICATES 

careful  attention  in  the  management  of  the  firing  than  on  the 
materials  used,  and  for  this  reason  it  seems  very  unlikely  that 
such  effects  will  ever  be  produced  cheaply. 

Lustre  Decoration. — This  style  of  decoration,  so  well 
illustrated  by  the  Hispano-Moresque  pottery  (produced  by 
the  Moors  in  Spain  during  mediaeval  times),  has  been  revived 
of  late  years.  The  colouring  components  of  the  lustres, 
which  are  applied  to  surfaces  already  covered  with  glaze  or 
enamel  duly  fired  on,  are  essentially  preparations  of  silver 
and  copper.  The  lustre  is  fired  on  the  ware  in  a  kiln  at  a 
red  heat,  the  kiln  being  filled  with  smoke  when  the  full 
temperature  is  reached.  The  cuprous  oxide  formed  by 
reduction  provides  a  fine  red  background,  and  on  this  is 
often  formed  an  iridescent  film  of  copper  or  silver.  In  this 
case  also  practically  everything  depends  on  the  management 
of  the  firing. 

Decoration  by  Coating  with  Metal. — Gold,  silver,  and 
platinum  are  the  only  metals  used  for  this  purpose,  the  two 
latter  being  often  employed  to  modify  the  effects  produced 
by  gilding.  Details  as  to  application  will  be  found  in 
Sandeman's  "  Notes  on  the  Manufacture  of  Earthenware," 
and  in  books  devoted  to  the  decoration  of  ceramic  wares. 


STONEWARE. 

The  distinguishing  features  of  stoneware  are  imperme- 
ability combined  with  opacity,  though  the  degree  in  which 
these  properties  are  possessed  depends  on  the  firing  as  well 
as  on  the  composition.  Several  varieties  come  under  this 
heading. 

Ordinary  Stoneware  (often  termed  "  common  "  stone- 
ware) . — This  has  usually  a  coloured  argillaceous  body,  though 
sometimes  only  slightly  blue  or  yellow,  and  is  prepared  in  a 
pasty  or  dry  manner,  and  may  be  with  or  without  glaze. 
For  its  production  are  used  vitrifiable  clays  containing  fluxes 
(alkalies,  lime,  magnesia)  and  ferric  oxide.  In  a  reducing 
atmosphere  the  iron  oxide  would  be  reduced  to  ferrous  oxide, 
which  would  then  act  as  a  flux.  A  medium  vitrifiable  clay 


CERAMIC  INDUSTRIES  231 

suitable  for  making  stoneware  contains  about  68  to  75  per 
cent,  silica,  20  to  25  alumina,  2  to  10-5  lime  and  magnesia, 
and  3  to  5  alkalies,  with  sometimes  10  to  15  per  cent,  iron 
oxide.  Sometimes  a  fusible  clay  is  made  more  refractory 
by  addition  of  a  proportion  of  fireclay,  with  also  some  silica, 
if  necessary,  to  give  a  composition  similar  to  that  of  a  normal 
stoneware  clay.  Sometimes,  again,  a  similar  result  is  obtained 
by  adding  fluxes  to  a  fireclay,  preferably  finely  powdered 
felspar  or  pegmatite  (or  similar  material — Cornish  stone  for 
example)  ;  from  considerations  of  economy,  calcareous 
clays,  marls,  and  calcium  carbonate  are  used,  or  (better) 
blast-furnace  slag,  which  is  an  impure  calcium  silicate. 
Bodies  thus  prepared  contain  about  51  to  55  per  cent,  silica, 
19  to  22  alumina,  and  22  to  25  lime,  together  with  small 
amounts  of  iron  oxide,  magnesia,  alkalies,  and  manganese 
oxide.  The  difference  between  the  action  of  alkalies  and  of 
lime  as  a  flux  is  noteworthy.  lyime,  when  the  temperature 
of  decomposition  of  the  carbonate  is  attained,  has  a  tendency 
to  form  rapidly  throughout  the  mass  a  double  silicate  of 
lime  and  alumina,  with  perhaps  aluminates  of  lime  also,  so 
that  it  softens  soon  after  vitrification  commences,  and  the 
ware  may  easily  lose  its  shape.  On  the  other  hand,  alkalies, 
in  the  form  of  mica  or  felspar,  act  quite  differently  ;  the 
minerals  melt,  and  the  flux  only  gradually  acts  on  the 
surrounding  silica  and  alumina,  and  a  comparatively  long 
interval  after  the  beginning  of  vitrification  precedes  deforma- 
tion. In  such  bodies,  the  alkalies  are  about  2  per  cent,  and 
the  lime  6  to  10  per  cent.  The  colour  of  stoneware  is  due 
(apart  from  the  very  rare  presence  of  manganese  oxide)  to 
iron  oxide  and  to  the  firing  conditions.  Grey  or  bluish-grey 
colours  are  produced  in  a  body  containing  a  little  iron,  and 
fired  under  reducing  conditions — best  above  800°  C.  Yellow 
colour  is  due  to  very  little  iron,  or  to  iron  along  with  lime, 
fired  under  oxidizing  conditions,  and  in  like  circumstances 
a  larger  proportion  of  iron  oxide  without  lime  gives  a  brown 
or  even  black  colour.  A  regular  black  can  only  be  produced, 
without  the  use  of  manganese  oxide,  by  firing  under  reducing 
conditions  and  the  deposition  of  carbon  in  the  pores  before 


232  SILICA   AND  THE  SILICATES 

the  body  vitrifies ;  for  this  it  is  best  to  put  the  ware  in 
saggars  filled  with  powdered  charcoal  or  coke. 

Drain  Pipes  of  true  stoneware  may  be  made  from  a 
thinned  vitrifiable  clay,  or  from  a  mixture  of  fireclay  with 
felspar  or  pegmatite,  etc.  On  economical  grounds  a  slightly 
vitrifiable  clay  or  a  fireclay  is  sometimes  mixed  with  a 
fusible  calcareous  clay,  and  in  that  case  the  ware  is  rarely 
fired  to  vitrification  owing  to  the  great  risk  of  deformation, 
and  so  it  does  not  acquire  the  lull  degree  of  impermeability. 
The  same  result  would  follow  from  the  use  of  fireclay  alone, 
but  in  this  case  owing  to  want  of  flux  to  promote  complete 
vitrification. 

Straight  pipes  can  be  made  from  the  plastic  material  either 
on  the  potter's  wheel  or  by  expression ;  in  the  latter  case, 
the  socket  is  made  separately  from  part  of  a  similar  but 
larger  pipe,  and  joined  on  by  hand,  or  is  made  by  machine. 
The  last  of  these  three  methods  is  the  most  extensively  used. 

The  firing  takes  place  in  round  down-draught  kilns,  or 
in  semi-continuous  kilns,  or  continuous  kilns  with  several 
burning  spaces.  When  the  maximum  temperature  is  reached, 
sea-salt  is  thrown  into  the  kiln  ;  it  volatilizes  a  little  above 
800°  C.,  but  only  decomposes  towards  1200°  C.  in  contact 
with  silicious  material,  with  the  result  previously  mentioned 
in  connection  with  the  glazing  of  earthenware,  a  superficial 
coating  of  glaze  being  formed.  To  avoid  excessive  lowering 
of  temperature  the  salt  should  not  be  introduced  all  at  once, 
but  in  several  portions  at  intervals. 

Culinary  and  Chemical  Stoneware  is  mostly  made 
from  a  fairly  pure  vitrifiable  clay,  which  does  not  need  wash- 
ing. It  is  chiefly  made  on  the  wheel,  and  rarely  by  expression 
(as  cylindrical  parts  of  bottles  or  pots,  to  which  the  other 
parts  are  joined  on)  ;  where  necessary,  moulds  are  used. 
This  ware  is  usually  rough  or  salt-glazed,  but  sometimes  a 
cheap  glaze  is  applied  by  dipping,  01  brushing  on,  or  powder- 
ing on,  the  unfired  body  ;  only  alkaline-calcareous  glazes 
are  available,  and  for  cheapness  are  used  in  the  form  of 
blast-furnace  slag,  or  a  mixture  of  cinders,  lime,  and  sand. 

Fine  Stoneware. — This  was  an  English  invention,  and  is 


CERAMIC  INDUSTRIES  233 

still  extensively  made  in  this  country.  It  is  made  of  ball 
clay,  China  clay,  and  Cornish  stone,  with  sometimes  addition 
of  flint,  and  occasionally  without  the  China  clay,  though  the 
latter  makes  it  whiter.  These  white  bodies  contain  about 
70  to  75  per  cent,  silica,  20  to  25  alumina,  0-5  to  i  lime,  and 
3  to  5  per  cent,  alkalies.  The  preparation  of  the  body  and 
the  shaping  are  as  described  for  earthenware,  and  the  firing 
is  also  conducted  similarly. 

Mortar  Body  (sometimes  called  Wedgwood  ware  after  the 
inventor)  is  a  specially  hard  fine  stoneware  used  for  making 
mortars  and  pestles  for  grinding. 

Jasper  (also  invented  by  Wedgwood)  is  a  fine  stoneware 
in  which  Cornish  stone  is  largely  replaced  by  cawk  or  heavy 
spar  (barium  sulphate).  This  body  is  variously  coloured — 
blue  by  cobalt  oxide,  green  by  chromium  oxide,  etc. 

Black  Basalt,  or  Egyptian  black  (another  of  Wedgwood's 
inventions),  is  a  variety  of  fine  stoneware  consisting  of  ball 
clay,  calcined  ochre,  and  manganese  oxide. 

Fine  stoneware  is  often  left  dull.  It  is  sometimes  glazed 
by  smearing — a  mixture  of  sea-salt  with  red  or  white  lead, 
etc.,  being  put  in  the  saggars  with  the  ware.  The  lead 
volatilizes,  probably  as  chloride,  and  this  in  contact  with  the 
surface  of  the  ware  forms  lead  silicate,  which  glazes  it. 
Glazes  similar  to  those  for  earthenware  are  often  used,  con- 
taining alkalies,  boric  acid,  and  lead  oxide  (besides  alumina 
and  lime). 


ENGLISH  PORCELAIN,  OR  BONE  PORCELAIN. 

This,  as  the  name  implies,  was  invented  in  England 
(towards  the  middle  of  the  eighteenth  century),  and  is  still 
made  there  almost  exclusively.  Its  firing  temperature  is 
much  below  that  necessary  for  hard  porcelain,  and  this  not 
only  reduces  the  cost  of  manufacture,  but  also  makes  it 
easier  to  decorate,  and  permits  a  much  wider  range  of 
colours  to  be  employed.  It  is,  of  course,  not  so  hard  as  the 
chief  continental  porcelains,  but  the  chief  reason  why  it  is 
not  made  to  any  great  extent  elsewhere  lies  in  the  inherent 


234  SILICA   AND  THE  SILICATES 

difficulties  associated  with  the  manufacture,  for  the  range 
of  possible  composition  of  a  satisfactory  body  is  very 
limited,  and  extreme  care  is  necessary  to  avoid  heavy  losses 
in  the  firing.  John  Dwight,  of  Fulham,  who  took  out 
patents  (Eng.  pats.  164,  April  23,  1671,  and  234, 
June  12,  1684),  made  excellent  stoneware,  some  of  which 
approached  porcelain  in  character,  but  the  origin  of  English 
porcelain  was  probably  not  long  before  Heylyn  and  Frye's 
patent  (No.  575,  July  15, 1741),  and  that  of  the  same  Thomas 
Frye  (No.  649,  November  17, 1749) .  Frye  was  for  some  years 
manager  of  the  famous  old  china  works  at  Bow  (L/ondon) . 

It  is  only  quite  recently  that  it  has  been  definitely  estab- 
lished— by  Dr.  J.  W.  Mellor  and  his  collaborators  at  the  Stoke- 
on-Trent  Pottery  Laboratory — that  the  practical  limits  of 
composition  are  so  very  narrow.  The  normal  constituents 
are  China  clay,  bone-ash  (ground  calcined  bones),  and 
Cornish  stone,  the  best  proportions  being  apparently  44  or 
45  per  cent,  of  bone,  with  equal  weights  of  the  clay  and  stone. 
Any  material  departure  from  these  proportions,  or  at  any 
rate  from  the  relative  proportions  of  clay  and  stone,  invites 
trouble.  An  increased  proportion  of  clay  (whether  intro- 
duced as  such,  or  associated  with  the  stone)  tends  to  produce 
blue  or  brown  china,  which  coloration  is  known  to  arise 
through  transfer  of  some  of  the  phosphorus  from  the  bone 
to  the  iron,  which  occurs  to  the  extent  of  about  i  to  if  per 
cent.,  in  a  bone  china  body.  This  may  be  otherwise  ex- 
pressed by  stating  that  the  presence  of  a  high  proportion  of 
alkalies  (in  the  stone)  prevents  the  transfer  of  phosphorus 
from  the  bone-ash  to  the  iron.  An  attempt  to  steer  clear 
of  this  difficulty  by  increasing  the  stone  (or  using  the  same 
proportion  of  a  softer  stone  which  contains  relatively  more 
alkali)  would  involve  serious  risk  of  blistering  or  bloating  of 
the  body.  The  composition  is  so  nicely  balanced  that  a 
change  only  from  a  softer  to  a  harder  variety  of  Cornish 
stone  may  cause  a  perfectly  white  body  to  become  blue 
(and  possibly  brown  later  through  oxidation,  or  conversely 
a  softer  stone  instead  of  a  harder  one  may  make  a  perfect 
body  liable  to  blister.  Even  a  variation  in  the  proportion 


CERAMIC  INDUSTRIES  235 

of  water  present  in  the  same  variety  of  stone,  or  in  the  China 
clay,  may  cause  similar  effects.  Indeed  all  three  varieties — 
blistered,  blue,  and  perfect  china — may  arise  from  the  same 
body  through  irregular  mixing.  It  is  remarkable  that  the 
proportion  of  bone  may  be  varied  considerably — and  variation 
of  the  water  present  in  it  is  accordingly  of  less  importance 
than  in  the  case  of  clay  or  stone — providing  the  amount  of 
China  clay  and  Cornish  stone  are  kept  nearly  equal.  Other 
important  facts  concerning  English  china  will  be  found  in 
papers  by  Dr.  J.  W.  Mellor  and  Mr.  Bernard  Moore  respec- 
tively, in  vol.  18  and  vol.  5  of  the  Transactions  of  the  Ceramic 
Society.  It  may  be  added  that  small  proportions  of  flint  or 
of  ball  clay  are  sometimes  used  in  the  body. 

The  body  for  bone  china  is  usually  prepared  by  the  wet 
method  as  for  earthenware  (though  with  some  differences  in 
detail) ,  but  it  is  much  less  plastic  than  earthenware.  Vertical 
pug-mills  are  generally  used.  As  regards  shaping,  much  of 
the  ware  is  made  on  the  potter's  wheel,  and  a  comparatively 
small  amount  is  produced  by  hand  pressing  in  plaster  moulds. 
Within  the  last  few  years  a  considerable  proportion  has  been 
shaped  by  casting. 

The  glaze  usually  contains  alkalies,  lime,  boric  acid,  and 
lead.  China  biscuit  being  at  most  only  slightly  porous,  the 
glaze  is  used  much  thicker  than  for  earthenware,  and  is 
applied  by  dipping.  Colour  decoration  may  be  applied 
underglaze  or  on-glaze,  including  lithographic  printing. 


FRENCH  SOFT  PORCELAIN. 

This  is  sometimes  called  "  fritted  "  porcelain  or  Reaumur 
porcelain,  and  was  the  first  type  of  porcelain  made  in  France. 
It  is  strictly  a  glass  (the  composition  being  very  like  that  of  a 
plate  glass,  except  the  relative  proportions  of  the  several 
bases),  but  is  made  like  pottery,  and  has  similar  appearance 
and  usage.  It  was  superseded  by  hard  porcelain,  but  its 
manufacture  has  been  revived  from  time  to  time.  The  body 
consists  of  a  mixture  of  75  per  cent,  of  a  glassy  frit,  with 
17  chalk  and  8  chalk  marl.  The  glassy  frit  contained 


236  SILICA   AND   THE  SILICATES 

60  per  cent,  silica,  the  remainder  being  chiefly  potash  and 
soda,  with  small  amounts  of  alumina  and  lime.  The 
plasticity  of  the  body  was  so  slight  that  it  was  mixed  with 
water,  soft  soap,  and  parchment  size,  before  it  could  be 
shaped  in  plaster  moulds  by  compression  with  a  plaster 
chuck.  The  piece  was  finished  (when  dry)  by  turning.  The 
firing  resembled  that  of  bone  porcelain  and  earthenware, 
consisting  of  a  oiscuit  firing  at  a  relatively  high  temperature, 
and  an  easier  glost  firing.  The  glaze  consisted  of  silicates 
of  lead,  potash,  and  soda,  and  was  applied  by  pouring  on. 
Owing  to  excessive  contraction  of  the  body,  the  pieces  had 
to  be  supported  by  suitable  props  made  of  the  raw  bod}7. 
Colouring  was  on-glaze  only. 

New  Sevres  Porcelain  (introduced  in  1884)  has  a  body 
somewhat  like  the  preceding,  but  even  richer  in  lime.  The 
biscuit  firing  is  to  cone  9.  The  glaze  differs  from  the  previous 
glaze  in  having  lime  instead  of  lead  oxide,  and  also  in  being 
much  richer  in  silica  ;  it  is  applied  by  dusting  or  by  dipping. 
The  decoration  may  be  either  underglaze,  on-glaze,  or  with 
coloured  glazes,  and  can  include  the  use  of  copper  blues  and 
iron  reds. 

SEGER  PORCELAIN. 

This  is  an  argillaceous  soft  porcelain,  and  is  an  imitation, 
both  as  to  body  and  glaze,  of  Japanese  porcelain.  It  is 
made  of  30  per  cent,  felspar,  35  to  40  quartz  sand,  and  the 
remainder  clay  or  kaolin  or  both.  The  glaze  has  the  same 
composition  as  Seger  cone  4  (ot3K2O.07CaO.o>5Al2O3.4SiO2, 
prepared  from  35  parts  whiting,  83*5  felspar,  26  China  clay, 
and  54  flint) .  The  biscuit  firing  is  very  easy,  and  the  glost 
firing  is  to  cone  9.  Though  composed  of  the  same  materials 
as  hard  porcelain,  the  greater  proportion  of  felspar  enables 
it  to  be  fired  at  a  much  lower  temperature,  but  in  hardness 
and  some  other  important  properties  it  is  very  much  inferior 
to  hard  porcelain. 

PARIAN. 

Parian  is  really  a  variety  of  soft  porcelain  which  is 
generally  left  without  glaze.  Any  soft  porcelain  could  be 


CERAMIC  INDUSTRIES  237 

so  employed,  but  in  most  cases  the  appearance  would  not 
be  satisfactory,  the  surface  being  too  dull  or  otherwise 
deficient. 

In  England,  bone  porcelain  for  this  purpose  has  been 
replaced  by  a  special  composition  (first  prepared  by  Copeland 
in  1 848),  which  fires  to  a  slightly  yellowish  tint  and  with  a 
waxy  surface  ;  the  name  parian  is  derived  from  its  resem- 
blance to  marble  from  Paros.  The  body  is  prepared  mostly 
from  a  mixture  of  China  clay  (kaolin)  and  felspar  or  Cornish 
stone,  with  or  without  small  proportions  of  glassy  material. 
It  may  be  coloured  to  imitate  various  natural  stones. 
Carrara  is  less  translucent  than  parian,  and  is  thus  inter- 
mediate between  the  latter  and  stoneware  ;  it  is  so  called 
from  its  resemblance  to  the  famous  Tuscan  marble.  Parian 
is  often  shaped  by  casting,  because  of  its  slight  plasticity. 
It  is  fired  in  England  at  the  same  temperature  as  for  bone 
porcelain,  and  owing  to  great  contraction,  as  well  as  softening, 
projecting  parts  are  kept  in  position  by  supports  made  of 
the  same  raw  body.  Parian  is  much  used  for  statuettes. 

HARD  PORCELAIN. 

This  constitutes  the  main  type  of  porcelain  made  on  the 
Continent,  and  also  in  China  and  Japan.  The  terms  "  hard  " 
and  "  soft/'  as  applied  to  porcelain,  indicate  that  the  former 
requires  a  greater  heat  in  firing,  and  also  offers  greater  re- 
sistance to  heat ;  they  also  indicate  a  corresponding 
relationship  as  regards  the  effective  mechanical  hardness  of 
the  glaze  on  the  surface  of  the  porcelain. 

Hard  porcelain  is  generally  made  of  kaolin  (sometimes 
two,  or  even  three  kinds),  quartz,  and  felspar,  the  quartz 
and  felspar  being  often  introduced  as  pegmatite  (or  similar 
granitic  rock),  and  in  some  cases  a  small  proportion  of  lime 
(as  carbonate)  is  included.  In  strong  contrast  with  soft 
porcelain,  hard  porcelain  has  a  very  easy  biscuit  firing  (in 
an  upper  chamber  of  the  oven,  at  about  800°  C.),  followed 
after  glazing  by  a  hard  firing  at  a  temperature  ranging  from 
about  1300°  to  1400°  C.,  or  even  higher,  to  mature  the  body 


238  SILICA   AND   THE  SILICATES 

and  glaze  together.  The  biscuit  firing  in  this  case  is  only 
intended  to  enable  the  ware  to  bear  manipulating  for  glazing, 
etc.  As  the  firing  is  conducted  under  reducing  conditions, 
the  porcelain  acquires  a  faint  bluish  white  tint.  The  glaze 
of  this  hard  porcelain  cannot  be  scratched  by  steel. 

Messrs.  Mintons  at  Stoke-on-Trent,  about  70  years  ago, 
made  hard  porcelain  from  a  mixture  of  100  parts  by  weight 
of  China  clay,  5  ball  clay,  and  20  felspar,  according  to 
(W.  G.  Turner  and  Herbert  Minton's)  Eng.  pat.  8124, 
June  22,  1839.  This  manufacture  was  under  the  super- 
intendence of  the  late  Mr.  Arnoux,  who  was  the  son  of  a 
French  hard  porcelain  manufacturer,  and  the  ware  produced 
was  of  good  quality.  The  loss  in  firing  was  excessive, 
however,  mainly  owing  to  defective  saggars — though  they 
were  made  of  ball  clay  from  Dorsetshire  mixed  with  grog 
from  similar  saggars — and  the  production  was  discontinued. 

Probably  China  clay  would  have  answered  much  better 
for  the  saggais,  if  the  difficulty  of  shaping  could  have  been 
surmounted  (as  by  use  of  a  proportion  of  ball  clay  to  give 
greater  plasticity). 

lyong  before  this  brief  effort  of  Minton's,  hard  porcelain 
was  made  in  this  country  (first  at  Plymouth)  by  William 
Cook  worthy,  the  discoverer  of  China  clay  and  Cornish  stone. 
Cookworthy  took  out  a  patent  (No.  898,  March  17,  1768)  for 
his  porcelain,  and  after  a  few  years  sold  it  to  Richard 
Champion,  of  Bristol,  who  obtained  an  extension  (No.  1096, 
September  15,  1775)  and  made  hard  porcelain  at  Bristol 
until  1781.  The  patent  was  then  sold  to  seven  vStaffordshire 
potters,  who  for  a  number  of  years  (until  1810)  made  an 
inferior  hard  porcelain  at  the  New  Hall  Works,  Hanley. 
The  moulds  and  trained  artists  from  Bristol  were  not  taken 
to  Staffordshire. 

During  the  last  two  or  three  years  an  experimental  in- 
vestigation of  continental  types  of  porcelain  has  been  in 
progress  at  Stoke-on-Trent  under  the  direction  of  Dr.  J.  W. 
Mellor  and  Mr.  Bernard  Moore,  and  early  in  1919  an  excellent 
assortment  of  products  manufactured  from  British  materials 
was  exhibited  locally,  and  it  is  understood  that  several 


CERAMIC  INDUSTRIES  239 

North  Staffordshire  firms  agreed  to  take  up  the  manufacture. 
These  products  are  of  an  intermediate  type,  which  has  been 
developed  by  the  use  of  materials  and  methods  at  hand, 
with  such  modifications  as  were  necessary  for  attaining  the 
object  in  view. 

The  glaze  used  for  continental  hard  porcelain  is  felspathic 
or  calcareous,  consisting  either  of  pegmatite,  with  or  without 
addition  of  calcium  carbonate — as  in  Germany  and  Austria — 
or  similar  compositions  made  up  from  other  materials  (felspar, 
quartz,  and  kaolin).  Quartz  is  sometimes  added  to  the 
pegmatite  to  regulate  its  fusibility.  The  glaze  is  generally 
applied  by  dipping,  but  very  fragile  pieces  are  glazed  by 
spraying  or  powdering.  The  glazed  ware  is  fired  in  the 
lower  chamber  of  the  oven,  the  waste  heat  being  to  some 
extent  utilized  for  baking  the  biscuit  ware  in  the  upper 
chamber. 

TOBACCO  PIPES. 

Clay  tobacco-pipes  are  made  from  pure  white  pipeclay, 
which  is  practically  free  from  such  impurities  as  sand,  iron 
compounds,  and  calcium  carbonate.  They  are  white  after 
being  fired.  In  some  places  clays  are  used  which  contain  a 
little  iron,  but  the  colouring  effect  of  the  iron  is  counter- 
acted by  the  presence  of  some  lime.  The  clay  does  not 
need  washing,  but  is  made  into  a  pasty  mass  by  hand,  and 
the  body  so  obtained  is  moulded  into  pipes  by  hand.  A 
small  piece  of  the  clay  is  well  kneaded,  and  then  rolled  out 
to  nearly  the  form  and  size  of  a  pipe,  a  small  lump  being 
left  at  one  end  to  represent  the  bowl.  After  a  short  time 
the  stem  part  of  the  roll  is  bored  by  means  of  a  needle  fixed 
in  a  wooden  handle  and  ending  in  a  small  rounded  enlarge- 
ment. The  head  or  bowl  portion  is  bent  to  the  proper  angle, 
and  the  whole  is  next  placed  in  one-half  of  a  copper  mould, 
the  other  half  being  then  put  on.  The  two  parts  are  pressed 
together  by  means  of  screws  and  nuts,  the  surfaces  of  the 
mould  being  previously  oiled  to  facilitate  delivery.  The 
head  or  bowl  is  partly  shaped  by  the  finger,  and  completed 
by  introducing  a  chuck  and  turning  it  round.  The  bottom 


240  SILICA   AND  THE  SILICATES 

of  the  bowl  is  pierced  by  means  of  a  point  so  as  to  com- 
municate with  the  bore  in  the  stem.  The  pipe  is  removed 
from  the  mould,  any  excess  of  clay  is  cut  away,  and  the  pipe 
is  smoothed  by  an  iron  or  copper  blade.  The  best  pipes  are 
polished  on  the  outside  by  rubbing  with  perforated  flints, 
and  the  needle  is  withdrawn.  The  shaping  is  usually  done 
by  women  or  children,  who  acquire  great  dexterity.  The 
pipes  are  dried  very  slowly,  and  finally  fired  either  in  muffle 
kilns,  or  in  saggars  in  small  round  or  square  kilns. 

For  pipes  made  in  two  pieces,  the  bowl  is  made  of  a 
plastic  ferruginous  clay,  thinned  when  necessary  with  grog 
made  of  the  same  clay.  The  bowls  are  shaped  by  throwing 
and  turning,  and  the  part  for  fixing  the  pipe  in — which  is 
turned  separately — is  then  stuck  on.  The  firing  is  done  in 
saggars.  The  chief  English  localities  for  pipe  making  are 
IvOndon  and  Broseley  (Shropshire). 

COMMON  POTTERY. 

This  includes  the  most  ancient  wares  made  in  most 
countries,  and  such  wares  were  for  a  long  period  almost 
entirely  used  for  all  domestic  purposes.  The  body  consisted 
of  a  fine-grained  fusible  clay,  or  of  clay  mixed  with  fine- 
grained sand  when  no  suitable  natural  clay  was  available. 
The  body  was  prepared  by  treading  and  kneading,  and 
shaping  was  done  by  hand  (without  moulds)  by  using 
pellets  for  large  pieces,  or  else  by  turning  (when  round). 
The  ware  was  fired  in  horizontal  kilns  with  a  single  fire-place, 
or  in  square  kilns  with  rising  flames.  The  very  fusible  clays, 
usually  both  ferruginous  and  calcareous,  employed  for  making 
these  wares,  are  still  referred  to  as  potters'  clays. 

TERRA-COTTA. 

This  term  is  generally  applied  to  fired,  unglazed,  yellow 
and  red  clay  wares.  Vases,  statues,  and  other  decorative 
objects  were  made  from  a  very  early  period,  especially  by  the 
Greeks,  but  during  the  Renaissance  the  manufacture  dis- 
appeared owing  to  the  introduction  of  faience,  and  it  was 


CERAMIC  INDUSTRIES  24! 

only  from  the  nineteenth  century  that  the  manufacture  of 
such  decorative  objects  was  resumed,  though  with  refine- 
ments. For  modern  terra-cotta  the  clays  are  washed,  and 
only  very  fine  sands  are  mixed  with  them  to  give  them  an 
open  texture,  so  that  the  surface  may  be  very  smooth.  The 
pieces  are  made  by  turning  or  moulding  by  hand  in  plaster 
moulds.  Statues  are  made  in  several  pieces,  which  are 
afterwards  carefully  stuck  together ;  in  this  way  all  the 
thicker  parts  (head,  trunk,  etc.)  are  kept  hollow,  and  possible 
evils  in  drying  and  firing  parts  of  very  uneven  thickness  are 
largely  avoided.  The  surface  may  be  decorated  by  stamping 
or  beading,  etc.  Uniformity  of  colour  is  important,  and 
according  to  the  clays  used  the  colour  may  be  yellowish- 
white,  yellow,  pink,  orange,  or  red,  and  by  mixing  manganese 
oxide  with  the  body,  grey  or  brown  colours  can  be  obtained. 
The  uniform  tint  is  more  easily  obtained  by  applying  to  the 
ware  thin  body  slip.  Coloured  slips  can  be  used  for  barbotine 
painting.  The  goods  are  usually  fired  in  saggars  or  in  a 
muffle.  Terra-cotta  wares  are  made  in  some  tile  works  and 
potteries,  and  also  in  special  works  as  at  Watcombe  (near 
Torquay).  Architectural  terra-cotta  is  made  from  similar 
materials,  which  in  this  case  are  particularly  required  to  be 
fine-grained  to  render  the  ware  better  able  to  withstand 
weather  influences. 

TH,ES. 

Decorative  Tiles  are  really  earthenware  slabs  variously 
decorated,  but  are  prepared  by  dry  pressing  (as  distinguished 
from  plastic  pressing).  The  body  is  prepared  in  the  same 
way  as  for  ordinary  earthenware.  The  clay  from  the  filter 
press  is  dried  in  a  steam  shed  until  quite  white,  and  is  then 
reduced  to  a  fine  powder  in  a  disintegrator  or  by  passing 
between  rollers.  The  dust  is  slightly  damped  (so  that  if 
squeezed  in  the  hand  it  sticks  together  somewhat  loosely), 
and  is  then  put  in  a  metal  mould,  covered  with  a  die,  and 
subjected  to  great  pressure  in  a  screw  press.  In  this  way  a 
tile  is  stamped  or  pressed  out  of  dust,  and  can  be  safely 
manipulated  until  it  becomes  fired,  after  which  it  is  hard 

c.  16 


242  SILICA   AND   THE  SILICATES 

and  sonorous.  The  glazing  of  decorative  tiles  is  sometimes 
effected  automatically  by  means  of  special  machines,  instead 
of  ordinary  dipping.  The  decoration  is  applied  in  the  same 
way  as  for  earthenware  objects. 

Decorative  tiles  are  largely  used  for  walls,  and  domestic 
hearths  and.  fire-places  ;  they  are  sometimes  framed  like 
pictures,  or  mounted  to  serve  as  stands  for  teapots,  etc. 

Roofing  Tiles. — These  require  a  more  plastic  body  than 
is  needed  for  bricks.  Formerly  they  were  exclusively 
moulded  by  hand.  Some  are  now  made  by  expression  of  a 
semi-stiff  body,  which  is  then  cut  into  tiles  of  definite  size 
and  shape  by  special  cutting  machinery.  Some  are  moulded 
in  a  press  from  semi-stiff  material  or  from  stiff  material. 
The  drying  of  the  tiles  generally  takes  place  in  large  ventilated 
buildings  over  the  kilns,  waste  heat  from  which  is  thus 
utilized.  In  rare  cases  independent  drying  sheds  are  used. 

During  the  firing  of  roofing  tiles,  the  fuel  or  ashes  should 
be  kept  out  of  contact  with  the  tiles. 

The  colour  of  black  tiles  may  be  produced  by  mixing 
manganese  oxide  with  the  body,  by  applying  black  slip  on 
the  body,  by  dipping  the  heated  body  in  hot  tarry  material,  by 
"  blueing  "  or  firing  the  tiles  in  a  reducing  atmosphere 
charged  with  hydrocarbons,  or  by  firing  the  tiles  in  saggars 
containing  powdered  carbon.  "  Blueing "  is  done  by 
introducing  tar  or  other  liquid  hydrocarbons  into  the  kiln 
after  it  has  reached  its  highest  temperature.  In  the  absence 
of  iron,  or  if  iron  be  present  combined  with  lime,  the  colour 
is  a  more  or  less  dirty  greyish  black,  but  under  other  con- 
ditions ferric  oxide  is  reduced,  giving  a  fine  blue  metallic 
gloss. 

Paving  Tiles. — Paving  tiles  or  floor  tiles  may  be  terra- 
cotta or  stoneware.  Terra-cotta  paving  tiles  are  usually 
square  or  hexagonal,  and  they  are  made  from  the  same  body 
as  roofing  tiles,  and  are  shaped  in  the  same  way — by  hand, 
or  expressed  and  cut  to  size.  The  firing  is  also  done  as  for 
roofing  tiles. 

Stoneware  paving  tiles  are  also  hexagonal,  square, 
rectangular,  etc.  They  may  be  white,  red,  brown,  black,  or 


CERAMIC  INDUSTRIES  243 

yellow  ;  or  they  may  be  decorated  with  inlaid  coloured 
slips  (encaustic  tiles).  As  in  the  case  of  other  stoneware 
products,  they  are  made  of  verifiable  clay,  etc.  They  are 
moulded  either  from  plastic  or  dry  body,  in  the  former  case 
as  for  terra-cotta  paving  tiles,  but  re-pressing  them  after 
partial  drying.  Thick  tiles  are  almost  exclusively  made 
by  pressing  more  or  less  dry  dust,  and  this  is  also  the  case 
with  inlaid  tiles  (encaustic  tiles).  The  body  for  encaustic 
tiles  may  be  a  vitrifiable  clay,  or  a  fireclay  mixed  with  felspar 
(or  Cornish  stone),  some  silica  being  added  when  necessary. 
It  is  decorated  by  coloured  slips  made  by  mixing  colouring 
oxides  with  the  body  ;  the  slips  are  applied  in  the  form  of 
powder  at  the  time  the  tile  is  made.  For  this  purpose  a 
copper  template  is  put  in  the  bottom  of  the  mould,  and  work- 
people then  in  their  turns  put  the  different  coloured  slips 
(powdered)  into  the  proper  places ;  the  template  is  then 
removed,  and  the  mould  is  filled  up  with  ordinary  body. 
The  whole  tile  is  then  subjected  to  pressure  in  the  usual  way. 
After  drying,  the  tiles  are  placed  in  saggars  and  fired. 

BRICKS. 

Bricks  constitute  a  variety  of  terra-cotta  used  for  building 
walls,  just  as  tiles  are  mostly  terra-cotta  used  for  roofs, 
floors,  etc.  The  word  brick  has  been  variously  derived  from 
Anglo-Saxon  brice  ("  a  fragment  "),  or  the  Teutonic  brikan 
("to  break"),  through  the  French  brique  (which  originally 
meant  "  a  broken  piece,"  especially  of  bread,  but  afterwards 
also  of  clay). 

Bricks  were  made  by  all  ancient  civilized  nations.  Well- 
fired  bricks  were  made  by  the  Babylonians  more  than  6000 
years  ago,  and  enormous  mounds  of  bricks  mark  the  sites 
of  the  walls,  towers,  and  palaces  of  Babylon.  The  Baby- 
lonians and  Assyrians  are  known  to  have  made  bricks 
coated  with  coloured  glazes  or  enamels.  The  Chinese  do 
not  seem  to  have  made  fired  bricks  when  they  were  made  by 
the  Babylonians.  The  great  wall  of  China  was  made  of 
bricks  which  had  apparently  been  only  sun-dried,  as  the 
material  turned  red  when  ignited.  J.  C.  Witt  (Philippine 


244  SILICA   AND    THE  SILICATES 

J.  Sci.,  I2A.,  257,  1918)  found  that  samples  of  brick  gave  on 
analysis  73*02  silica,  18*96  alumina  and  iron  oxide,  5*73 
alkalies,  1*29  lime,  1*05  magnesia,  with  0*10  loss  on  ignition, 
and  the  mortar  gave  48*83  lime,  4*03  magnesia,  0*85  alkalies, 
2'i2  silica,  0*44  alumina  and  iron  oxide,  with  43*88  loss  on 
ignition,  so  that  no  sand  had  been  mixed  with  the  mortar. 
The  bricks  made  in  Egypt  by  the  Israelites  were  probably 
also  only  sun-dried  ;  they  were  made  of  Nile  mud  (clay) 
mixed  with  chopped  straw  or  reeds  and  water  to  a  stiff  paste. 
The  chopped  straw  (or  reeds)  was  added  as  a  binding 
material,  but  it  also  increases  the  plasticity  of  clay,  especially 
after  standing  a  few  days.  Sun-dried  bricks — called 
"  adobes  " — are  still  made  from  Nile  mud  mixed  with  chopped 
straw  and  water  ;  the  material  is  trampled  to  effect  thorough 
mixing,  and  the  bricks  are  shaped  in  moulds  or  by  hand. 
The  Greeks  seem  to  have  used  bricks  only  to  a  limited  extent, 
but  the  Romans  took  up  the  manufacture  of  bricks  on  an 
extensive  scale,  selecting  and  preparing  the  clay  with  great 
care,  and  they  introduced  the  process  of  firing  bricks  in  kilns, 
After  the  withdiawal  of  the  Romans  from  Britain,  bricks 
did  not  come  into  general  use  again  until  the  fifteenth 
century,  and  then  only  for  buildings  of  importance.  Even 
to  the  time  of  the  Great  Fire  of  London  (in  1666),  the  less 
important  buildings,  as  well  as  shops  and  dwelling-houses, 
were  made  with  a  timber  framework  in  combination  with 
lath  and  plaster  work.  When  I/ondon  was  rebuilt  after  the 
fire,  bricks  were  used  extensively,  and  from  the  later  years 
of  the  seventeenth  century  onward  they  have  been  generally 
used  in  all  ordinary  buildings  except  where  building  stone 
is  abundant  and  good  clay  for  bricks  is  not  readily  available. 
Since  1625  the  sizes  of  bricks  have  been  regulated  by  statute, 
the  standard  adopted  being  9  X4^X3  in. 

The  clays  used  for  making  bricks  until  modern  times 
were  only  the  alluvial  and  drift  clays  found  at  or  near  the 
surface,  which  are  easy  to  work,  and  do  not  need  much 
preparation.  Alluvial  clay  is  merely  accumulated  river 
mud  which  is  soft  and  plastic.  Within  comparatively  recent 
times,  hard  clays,  shales,  and  slates  have  been  brought  into 


CERAMIC  INDUSTRIES  245 

use  for  making  bricks,  with  the  aid  of  heavy  machinery. 
These  hard  clays  or  shales,  etc.,  may  be  rendered  plastic  by 
prolonged  weathering — exposure  to  rain,  frost,  and  sun — 
or  by  crushing  and  grinding  in  water ;  they  then  closely 
resemble  ordinary  alluvial  clays. 

Brick  earths  or  clays  fall  into  two  chief  classes  :  (i)  non- 
calcareous  clays  or  shales,  with  little  calcium  carbonate 
(carbonate  of  lime),  composed  mainly  of  true  clay  substance 
with  sand,  felspar  grains,  and  iron  compounds  (chiefly  oxide 
or  carbonate),  and  which  when  fired  usually  become  buff, 
salmon,  or  red ;  (2)  calcareous  clays,  containing  (besides 
the  substances  just  mentioned)  a  considerable  amount — 
up  to  40  per  cent. — of  calcium  carbonate  ;  these  latter  are 
called  marls,  and  when  fired  they  assume  a  characteristic 
pale  yellow  tint.  It  may  be  noted  here  that  the  term  ' '  marl ' ' 
is  often  wrongly  applied  to  fireclays,  but  should  be  confined 
to  natural  mixtures  of  clay  and  chalk,  such  as  those  found 
in  parts  of  the  Thames  valley,  and  in  other  regions  where 
chalk  or  limestone  formations  occur. 

Brick  clays  of  the  first  class  are  very  widely  distributed. 
Many  natural  clays  are  unsuitable  for  making  bricks 
unless  mixed  with  some  clay  of  a  different  kind  or  with 
sand. 

All  clays  contain  more  or  less  free  silica  as  sand,  and 
generally  also  a  little  felspar.  Next  in  importance  to  the 
clay  substance  and  the  sand  is  iron  oxide,  for  the  colour  of 
the  fired  bricks  depends  on  this  component.  The  iron  oxide 
in  the  clays  ranges  from  about  2  to  10  per  cent.,  and  the 
colour  of  the  corresponding  bricks  varies  from  light  buff  to 
chocolate  brown.  It  should  be  borne  in  mind,  however, 
that  the  colour  produced  by  the  same  proportion  of  iron 
oxide  may  be  modified  by  the  presence  of  alkalies  or  other 
substances,  and  also  may  be  very  different  according  to  the 
manner  of  firing.  Thus  red  bricks  are  ordinarily  produced 
from  clays  containing  5  to  8  per  cent,  of  iron  oxide,  but  if 
the  bricks  are  fired  too  hard,  or  if  the  clay  contains  also 
3  to  4  per  cent,  of  alkalies,  a  darker  colour  (more  purplish) 
will  be  produced.  Excess  of  alumina,  on  the  other  hand, 


246  SILICA   AND   THE   SILICATES 

tends  to  produce  lighter  and  brighter  colours.  The  com- 
position of  such  clays  varies  widely,  the  clay  substance 
being  from  15  to  80  per  cent.,  sand  (free  silica)  5  to  80,  iron 
oxide  i  or  2  to  10,  calcium  carbonate  (including  magnesium 
carbonate)  i  to  5,  and  alkalies  i  to  4  per  cent.  There  is 
always  some  organic  matter  present,  and  often  calcium  and 
magnesium  sulphates,  sodium  and  potassium  chlorides  and 
nitrates,  and  iron  pyrites,  all  except  the  organic  matter  and 
the  iron  pyrites  being  soluble  in  water.  Iron  pyrites,  when 
present  in  sufficient  quantity,  leaves  iron  sulphide  after 
the  firing,  and  this  soon  changes  to  sulphate,  which  before 
very  long  weathers  out,  and  makes  the  brick  brittle.  The 
organic  matter  increases  the  plasticity  of  the  clay,  but  some- 
times it  is  present  in  excessive  amount.  The  soluble  sub- 
stances are  objectionable  because  they  form  a  superficial 
scum,  which  causes  patchy  colour  and  pitted  faces  of  the  bricks. 
The  most  frequent  of  these  soluble  impurities  is  calcium 
sulphate,  which  causes  a  whitish  scum  on  the  brick's  face 
when  drying  ;  this  scum  is  fixed  by  the  filing,  and  makes 
the  brick  fit  only  for  common  work.  When  a  clay  containing 
calcium  sulphate  has  to  be  used  for  making  high-class  facing 
and  moulded  bricks,  a  suitable  amount  of  barium  carbonate 
is  mixed  with  the  wet  clay,  resulting  in  the  formation  of 
insoluble  calcium  carbonate  and  barium  sulphate ;  the 
impurities  thus  remain  distributed  through  the  brick  instead 
of  the  soluble  substance  collecting  on  the  surface.  Magnesium 
salts  are  also  undesirable  in  clays,  as  they  mostly  remain  in 
the  fired  bricks  as  magnesium  sulphate,  and  this  may  later 
cause  an  outgrowth  of  fine  white  crystals  on  the  surface. 
Mica  and  felspar,  with  iron  oxide,  act  as  fluxes,  and  if  not  in 
excess  are  useful  rather  than  injurious  in  brick  clays. 

Strong  or  plastic  clays  are  often  termed  "  fat  "  clays  ; 
they  are  always  rich  in  clay  substance,  and  contain  little 
sand.  Fat  clays  take  up  a  relatively  large  amount  of  water 
when  tempered  ;  they  then  dry  slowly,  show  great  shrinkage. 
and  so  are  very  liable  to  lose  their  shape  and  to  crack  during 
the  drying  and  firing.  The  addition  of  coarse  sharp  sand 
much  improves  fat  clays,  by  reducing  the  time  required  for 


CERAMIC  INDUSTRIES  247 

drying,  by  reducing  also  the  shrinkage,  and  by  enabling  the 
bricks  to  stand  more  firmly  during  the  firing.  Coarse  sand 
is  practically  not  changed  during  the  drying  and  firing,  and, 
if  not  absolutely  necessary,  is  a  desirable  component  of  all 
brick  clays.  Hence  the  best  brick  clays  feel  gritty,  but 
should  be  plastic  enough  to  be  shaped,  and  strong  enough 
for  safe  handling  after  drying.  Weathering  or  ageing— by 
being  turned  over  and  thoroughly  exposed  to  the  weather, 
or  by  being  left  to  stand  several  months  in  a  damp  state — 
greatly  improves  all  clays  by  increasing  plasticity  and  making 
them  more  uniform. 

Calcareous  clays  or  marls  are  not  so  common  as  ordinary 
brick  clays,  and  natural  deposits  of  them  in  England  have 
been  mostly  used  up.  A  very  fine  "  malm/'  as  the  chalky 
clay  was  termed  locally,  was  obtained  from  the  alluvium 
about  Ivondon,  and  an  artificial  malm  is  now  made  by  mixing 
an  ordinary  brick  clay  with  ground  chalk.  For  making 
best  lyondon  facing  bricks,  the  chalk  is  ground  on  pans,  and 
the  clay  is  mixed  with  water  to  a  creamy  consistency  ;  the 
mixture  of  the  two  is  run  through  a  grating  or  coarse  sieve 
on  to  a  drying  kiln  or  "  bed/'  and  when  stiff  enough  the 
mass  is  covered  with  fine  ashes,  after  which  it  is  turned  over 
and  mixed  by  a  spade,  and  tempered  by  adding  water. 

Where  clays  containing  limestone  are  used,  the  marl  is 
simply  mixed  with  water  on  a  wash-pan,  and  the  creamy 
mixture  is  passed  through  coarse  sieves  on  to  a  drying  bed. 
When  necessary  or  desirable,  coarse  sand  is  added  to  the 
marl  in  the  wash-pan.  The  plastic  marls  are  sometimes 
squeezed  (by  machinery)  through  iron  gratings,  which 
separate  pebbles.  Alternatively  the  marl  is  passed  through 
a  grinding-mill  with  solid  bottom  and  heavy  iron  rollers,  the 
limestone  pebbles  being  thus  crushed  and  distributed  through 
the  whole  of  the  material.  If  not  removed,  the  limestone 
pebbles  would  be  converted  by  the  heat  into  quicklime,  and 
this  would  be  liable  to  cause  shattering  of  the  brick  on 
exposure. 

As  already  intimated,  these  marls — usually  containing 
15  to  30  per  cent,  calcium  carbonate — after  firing  exhibit  a 


248  SILICA   AND   THE  SILICATES 

distinct  pale-yellow  colour,  though  with  over  40  per  cent, 
calcium  carbonate  the  colour  becomes  grey  or  very  pale  buff. 
Marls  generally  show  little  or  no  contraction  on  burning, 
and  as  a  rule  produce  strong  bricks  possessing  fine  texture 
and  good  colour ;  when  the  marls  are  rich  in  calcium 
carbonate  they  produce  good  durable  bricks  at  a  very  low 
temperature.  Underfired  marl  bricks  have  a  strong  tendency 
to  disintegrate  when  exposed  to  the  weather ;  hence  care 
should  be  taken  to  avoid  underfiring. 

Making  Bricks. — The  tempered  clay  may  be  made  into 
bricks  by  hand  moulding  or  machine  moulding,  and  the 
machines  may  be  worked  by  hand  or  by  mechanical  power. 
The  clay  after  weathering  is  mixed  with  water  to  the  proper 
consistency,  and  any  sand  or  sandy  substance  used  is  then 
added  and  mixed  well  with  the  clay.  Pug-mills  are  some- 
times used  for  the  mixing.  The  moulds  may  be  of  wood,  of 
iron,  or  of  wood  strengthened  by  iron  bands,  and  may  be 
open  above  and  below  or  may  have  a  bottom  in  addition  to 
the  four  sides.  To  prevent  sticking,  the  mould  before  being 
used  is  either  dipped  in  water  or  dusted  over  with  sand, 
though  in  the  former  case  they  are  more  liable  to  split  in 
drying. 

The  moulding  by  hand  is  done  in  all  cases  by  knocking 
a  piece  of  the  prepared  clay  body  sharply  into  the  mould, 
taking  care  to  fill  the  corners,  and  afterwards  levelling  the 
upper  surface  with  a  scraper  or  strip  of  wood  called  a 
"  strike  "  ;  the  brick  is  then  turned  out  of  the  mould  on  to  a 
board  and  afterwards  carried  away  to  be  dried.  The  mould 
may  be  placed  on  a  piece  of  wood  (called  the  stock-board) 
to  form  the  hollow  or  "  frog  "  in  the  bottom  of  the  brick. 

A  soft  body  is  moulded  best  and  most  economically  by 
hand.  Though  the  ware  cannot  be  handled,  it  requires 
extensive  drying  space,  and  shiinks  very  much,  and  being 
very  porous  it  has  little  strength.  Machines  are  only  of 
advantage  with  stiffer  bodies,  which  produce  goods  that  can 
be  handled  more  readily,  piled  up,  and  dried  easily.  In 
nearly  all  cases  the  direct  expenses  of  working  with  machines 
will  always  be  higher,  but  the  total  expenses  of  manufacture 


CERAMIC  INDUSTRIES 


249 


can  be  reduced  by  well-considered  arrangements,  by  using 
suitable  raw  materials,  and  by  charging  extra  for  better 
quality.  To  improve  the  shape,  and  sometimes  to  modify 
it,  bricks  are  partly  dried  until  they  are  leather-hard,  and 
are  then  re-pressed,  the  machines  used  for  the  purpose  being 
worked  by  hand  or  by  power. 

Machine-made  bricks  may  be  classed,  according  to  the 
method  of  making,  as  plastic  or  semi-plastic,  but  the  same 
type  of  machine  is  often  used  for  both.  Bricks  made  of 


FIG.  19. — Stiff  plastic  machine  for  brick-making. 
(C.  Whittaker  &  Co.,  Ltd.) 

semi-plastic  clay — ground  clay  or  shale  damp  enough  to 
stick  together  under  pressure — are  generally  machine-made 
throughout.  Machine-made  plastic  bricks  are  made  of 
tempered  clay,  but  the  clay  is  generally  prepared  by  grinding- 
mills  and  pug-mills,  the  grinding-mills  being  either  a  series  of 
rollers  or  an  ordinary  edge-roller  pan.  Shales  are  sometimes 
crushed  before  exposure  for  weathering.  For  ordinary 
plastic  brick  clay,  grinding-mills  are  only  used  when  pebbles 
larger  than  a  quarter  of  an  inch  in  diameter  occur  ;  otherwise 


250  SILICA   AND  THE  SILICATES 

it  may  be  passed  at  once  through  the  pug-mill  (and  a  second 
time  if  necessary).  A  pug-mill  is  essentially  an  iron  box 
with  a  central  shaft  carrying  knives  or  cutters,  arranged  so 
that  when  the  shaft  revolves,  the  clay  fed  in  at  one  end  is 
cut  and  kneaded,  and  also  impelled  towards  and  through  a 
delivery  hole  at  the  other  end,  and  as  the  delivery  hole  has 
a  cross  section  approximately  equal  to  that  of  a  brick 
(9x4!  plus  the  amount  of  contraction  for  ordinary  bricks), 
the  issuing  clay  has  only  to  be  cut  at  proper  intervals  to 
make  bricks.  The  cutting  is  effected  by  means  of  a  wire- 
cutting  frame  which  cuts  off  many  bricks  together  at  regular 
intervals,  and  bricks  made  in  this  way  are  termed  "  wire- 
cuts."  These  wire-cuts  may  be  improved  by  subjecting 
them  to  great  pressure  under  a  brick-press,  which  is  often 
done  automatically  by  using  a  combined  pug-mill  and 
brick-press. 

The  essential  parts  of  a  brick-press  are  a  box  or  frame 
in  which  the  clay  is  to  be  moulded,  a  plunger  or  die  at 
the  end  of  a  ram  which  transmits  pressure,  and  a  con- 
trivance for  discharging  the  pressed  brick  from  the  mould- 
ing-box. 

Bricks  pressed  from  tempered  or  plastic  clay  are  the  best, 
but  the  manufacture  of  semi-plastic  or  dust-made  bricks 
has  been  much  developed,  especially  where  bricks  are  made 
from  shale.  These  semi-plastic  bricks  are  stamped  out  of 
moistened  ground  shale  (that  is,  coarsely  powdered  clay) 
much  in  the  same  way  as  dust-pressed  decorative  tiles  from 
prepared  powdered  white  clays.  The  method  is  applicable 
for  certain  hard  clays  or  shales  that  it  would  be  impracticable 
to  use  for  making  plastic  bricks  ;  weathering,  tempering, 
and  ageing  may  be  wholly  or  in  great  part  omitted.  The 
necessary  plant  is  heavier  and  more  costly,  but  the  processes 
are  simpler  than  with  plastic  clay. 

The  drying  of  bricks  generally  takes  place  in  a  special 
shed  heated  by  flues  leading  from  the  kilns  to  the  chimney. 
Ventilation  is  necessary  to  keep  the  drying-shed  fairly  dry. 
Too  moist  an  atmosphere  causes  brick  surfaces  to  keep  damp, 
so  that  water  comes  from  the  inside  to  the  surface  in  the 


CERAMIC  INDUSTRIES  251 

liquid  state,  and  dissolved  salts  are  left  on  the  surface  as  the 
water  evaporates  slowly,  thus  giving  rise  to  scum. 

Drying  in  the  open  air  is  only  used  for  hand-made 
bricks. 

Plastic  bricks  dry  much  more  slowly  than  semi-plastic 
bricks ;  they  also  show  more  shrinkage,  and  a  greater 
tendency  to  warp. 

The  firing  of  bricks  is  very  important,  the  strength  and 
durability  of  bricks  depending  in  great  measure  on  suitable 
firing.  Ordinary  bricks  should  be  fired  in  an  oxidizing 
atmosphere,  and  the  finishing  temperature,  which  (according 
to  the  nature  of  the  clay)  may  range  from  about  900°  to 
1250°  C. — and  is  usually  about  1050°  C.  for  ordinary  bricks — 
should  be  adjusted  so  as  to  produce  hard,  strong,  well- 
shaped  bricks,  not  too  porous,  and  resistant  to  frost.  Cal- 
careous clays  generally  require  a  firing  temperature  of  about 
1150°  to  1200°  C.  to  bring  about  chemical  combination 
between  the  lime  and  other  substances  present. 

Wherever  bricks  are  made,  the  old  plan  of  firing  in 
"  clamps  "  is  followed  in  the  smaller  works,  though  the 
Romans  used  permanent  kilns  before  the  Christian  era 
commenced.  Clamps  are  formed  by  building  up  the  unfired 
bricks  into  a  series  of  walls  arranged  rather  close  together 
so  as  to  constitute  a  rectangular  stack ;  channels  or  fire- 
mouths  are  formed  at  the  bottom  of  the  clamp,  and  during 
the  building  of  the  stack,  layers  of  fine  coal  are  spread 
horizontally  between  the  bricks.  In  the  fire-mouths  fires 
are  kindled,  and  the  clamp  is  left  burning  until  all  the  fuel 
is  burned  away,  after  which  it  is  left  to  cool.  The  cooled 
clamp  is  then  taken  down,  and  the  bricks  are  sorted.  Under- 
fired  bricks  are  built  up  again  in  the  next  clamp  to  be  re- 
fired.  In  some  cases  the  outside  of  a  clamp  is  built  of  fired 
bricks  with  clay  plastered  over  them,  and  the  fire-mouths 
are  made  larger  and  better.  The  method  of  building  up  the 
clamps  varies  as  regards  details  in  different  localities,  but 
the  aim  is  always  to  produce  a  large  proportion  of  well- 
fired  bricks.  Clamp  firing  has  the  disadvantages  of  being 
slow,  irregular,  and  imperfectly  controlled,  wherefore  it  is 


252  SILICA   AND   THE  SILICATES 

not  economical,  and  is  only  resorted  to  where  the  output  is 
small. 

Brick  kilns  are  of  many  forms,  but  all  are  included  in  two 
main  groups,  intermittent  kilns  and  continuous  kilns. 

Intermittent  brick  kilns  are  usually  round  with  a  domed 
top ;  sometimes  they  are  rectangular  in  shape.  Each  kiln 
consists  of  a  single  firing  chamber  in  which  the  dried  bricks 
are  arranged,  and  around  the  wall  are  placed  a  number  of 
fire-mouths,  in  which  fuel  in  the  form  of  coal  or  wood  is 
burned.  The  kilns  may  be  up-draught  or  down-draught. 
In  up-draught  kilns  (which  represent  the  earlier  type)  the 
combustion  products  pass  from  the  fire-mouths  through 
flues  below  the  floor  to  holes  in  the  floor,  and  so  into  the 
bottom  of  the  firing  chamber,  whence  they  pass  upwards 
and  escape  at  the  top.  In  the  modern  down-draught  kiln 
there  is  a  short  chimney  or  "  bag  "  connected  with  each  fire- 
mouth,  and  this  bag  directs  the  flames  and  hot  gases  to  the 
upper  part  of  the  firing  chamber,  whence  they  pass  down- 
wards among  the  bricks  and  through  holes  in  the  bottom 
leading  to  flues  connected  with  an  independent  chimney. 
The  bags  are  sometimes  run  together  so  as  to  form  a  con- 
tinuous passage  bounded  by  an  inner  circular  wall  all  round 
the  firing  chamber  except  across  the  doorway.  A  single 
chimney  may  serve  for  a  group  or  row  of  kilns.  Down- 
draught  kilns  are  more  economical  as  regards  fuel  consump- 
tion than  up-draught  kilns,  and  usually  fire  more  regularly 
and  give  a  larger  proportion  of  well-fired  bricks ;  some  of 
the  waste  heat  may  be  utilized  for  drying  purposes  by  taking 
the  flues  under  the  drying  shed  floor  on  the  way  to  the 
chimney. 

Continuous  kilns  may  have  one  or  several  firing  chambers. 
Those  with  a  single  firing  chamber  may  either  have  the  goods 
carried  gradually  past  fixed  fire-places,  or  the  fire-places  can 
be  put  in  the  middle  of  goods  which  are  fixed  in  position. 
The  first  of  these  methods,  after  many  trials,  has  been  given 
up  so  far  as  bricks  are  concerned.  In  the  second  kind,  the 
fires  can  be  spread  about  in  the  firing  chamber. 

A  chambered  continuous  kiln  really  consists  of  several 


CERAMIC  INDUSTRIES 


253 


kilns  or  firing  chambers,  built  in  series,  and  connected  with 
the  main  flue  to  the  chimney,  so  that  the  combustion  pro- 
ducts from  one  kiln  may  be  made  to  pass  through  other  kilns 
before  entering  the  main  flue.  The  first  continuous  kiln  of 
this  type  was  invented  by  Hoffmann  and  Licht,  in  1858,  and 
had  an  annular  firing  chamber  with  twelve  charging  doors, 
and  a  central  chimney,  the  latter  communicating  (through 
the  agency  of  a  smaller  annular  chamber  surrounding  it)  by 
twelve  passages  or  flues  with  the  firing  chamber.  A  movable 
damper  can  be  placed  near  any  door  so  as  to  quite  shut  off 


FIG.  20.—  ''  Staffordshire"  kiln.     Cross  section  showing  interior  of  cham- 
bers, arrangements  of  flues,  and  grate  fire.     (Dean  Hetherington  &  Co.) 

the  firing  chamber.  The  whole  kiln  consists  of  twelve 
chambers,  each  with  a  charging  door  at  one  end  and  a  draught 
passage  at  the  other,  though  there  is  no  actual  separation  of 
the  chambers  except  by  the  one  movable  damper.  In 
normal  working,  two  adjoining  chambers  are  being  charged 
and  emptied  respectively,  only  the  door  of  the  second  of 
these  being  open ;  the  four  chambers  next  to  the  latter  are 
cooling  after  firing,  the  next  two  are  getting  their  final 
heating  from  the  fire,  and  the  last  four  are  being  gradually 
heated  up.  The  air  for  combustion  enters  through  the  only 
open  door,  travels  towards  and  past  the  fire,  and  so  all  round 


254  SILICA   AND  THE  SILICATES 

the  firing  chamber,  first  helping  to  cool  chambers  which 
have  been  fired,  and  afterwards  assisting  in  the  heating  of 
the  other  chambers ;  from  the  last  compartment  it  makes 
its  way  through  the  smoke  flue  or  draught  passage  (all  the 
others  being  shut  off)  to  the  chimney.  As  the  firing  of  each 
chamber  is  finished,  the  movable  damper  is  shifted  to  the 
next  door  in  the  direction  of  the  cooling  ware,  and  so  on  in 
turn  all  round,  with  corresponding  adjustments  for  the 
draught,  etc.  Numerous  modifications  have  taken  place, 
the  chief  being  the  substitution  for  the  circular  firing  chamber 
of  a  rectangular  chamber  consisting  of  two  parallel  rows  of 
the  single  chambers,  with  communicating  flues  at  the  two 
ends,  and  the  chimney  generally  outside.  Continuous  kilns 
usually  give  more  uniformly  fired  products  than  intermittent 
kilns,  and  are  economical  in  fuel.  Some  continuous  kilns 


FIG.  21. — Dunnachie's  patent  continuous  regenerative  gas-fired  kiln. 
(Dean  Hetherington  &  Co.) 

are  divided  into  several  chambers,  and  the  products  of  com- 
bustion pass  from  one  chamber  through  flues  beneath  the 
floor  into  the  next,  and  so  on  successively. 

Gas  firing  is  now  frequently  applied  to  continuous  kilns. 
In  Mendheim's  gas-fired  continuous  kiln,  a  good  distribution 
of  the  heat  is  obtained  by  a  special  arrangement  of  the  gas 
and  hot-air  passages. 

Where  pyrometers  are  used  to  control  the  firing,  the 
products  are  much  more  constant  in  quality. 

Blue  Bricks  are  very  strong  vitrified  bricks  with  a  dark 
slaty -blue  colour.  These  blue  bricks  are  made  in  the  same 
way  as  other  bricks,  but  of  clay  containing  7  to  10  per  cent, 
iron  oxide,  and  when  during  the  firing  the  kiln  has  reached 
its  highest  temperature,  a  smoky  (and  therefore  reducing) 
atmosphere  is  produced  by  throwing  small  bituminous  coal 


CERAMIC  INDUSTRIES  255 

on  the  fires  and  shutting  out  air  as  much  as  possible.  The  red 
oxide  of  iron  is  thus  reduced  to  ferrous  oxide,  which  combines 
with  silica  to  form  a  fusible  ferrous  silicate,  the  latter  partly 
filling  up  the  pores,  forming  a  vitreous,  impermeable  layer 
the  thickness  of  which  is  dependent  on  the  duration  and  other 
features  of  the  smoking,  on  the  finishing  temperature  of  the 
kiln,  and  on  the  texture  of  the  bricks.  In  the  earlier  stages 
of  the  smoking,  carbon  particles  penetrate  the  brick,  and 
tend  to  darken  the  colour. 

Floating  Bricks  were  made  in  ancient  times,  but  the 
method  of  making  them  was  lost  for  centuries.  Fabroni 
rediscovered  it  in  1790,  and  made  them  from  the  kieselguhr 
(diatomaceous  earth  or  fossil  meal)  of  Tuscany.  Besides 
being  light,  these  bricks  are  fairly  strong,  and  are  poor 
conductors  of  heat.  This  last  property  enabled  them  to  be 
used  in  building  powder  magazines  on  ships,  etc. 

The  Moler  Bricks,  much  used  in  Norway,  are  of  this 
class.  They  are  made  by  mixing  the  (diatomaceous)  Moler 
earth  with  a  little  water,  passing  through  crushing  rolls, 
and  then  tempering  in  a  pan-mill.  The  bricks  are  moulded 
by  machines  or  by  extrusion  presses.  Some  have  been  made 
so  light  that  a  cubic  foot  of  the  material  only  weighed  350  oz., 
and  these  very  light  bricks  are  excellent  insulators. 

Mortar  Bricks,  which  are  not  fired,  are  really  blocks  of 
artificial  stone  made  in  brick  moulds.  They  are  made  of  a 
mixture  of  sand  and  slaked  lime,  which  is  moulded,  and  the 
blocks  are  left  to  harden  in  the  air.  The  hardening  results 
partly  from  evaporation  of  the  water,  but  chiefly  by  the 
action  of  atmospheric  carbon  dioxide  (carbonic  acid  gas)  on 
slaked  lime  to  form  calcium  carbonate.  A  small  proportion 
of  the  lime  combines  with  silica  and  water  to  form  a  hydrated 
calcium  silicate,  and  probably  a  little  hydrated  basic  calcium, 
carbonate  is  formed  as  well ;  both  these  latter  have  cemen- 
titious  properties.  The  natural  hardening  was  found  to 
take  6  to  1 8  months.  The  bricks  were  eventually  improved 
in  strength  by  adding  some  cement  (or  certain  ground  blast- 
furnace slags  mixed  with  lime),  and  by  using  hydraulic 
presses  for  moulding,  afterwards  treating  the  bricks  with 


256  SILICA   AND   THE  SILICATES 

carbon  dioxide  under  pressure.  Some  of  the  mixtures  and 
improved  methods  are  still  used,  but  sand-lime  bricks  have 
latterly  practically  superseded  the  old  mortar  bricks. 


BRICKS. 

These  were  first  made  according  to  the  patent  of  Michaelis 
nearly  forty  years  ago  in  Germany.  Blocks  made  of  a 
mixture  of  sand  and  lime  were  hardened  by  subjecting  them 
to  the  action  of  high-pressure  steam  for  a  few  hours.  They 
are  now  made  very  largely  in  many  countries,  especially  in 
districts  where  clays  suitable  for  ordinary  bricks  are  not 
available.  Subject  to  many  differences  in  detail,  the  general 
method  of  making  the  bricks  is  the  same  everywhere.  Dry 
sand  is  well  mixed  with  about  one-tenth  of  its  weight  of 
slaked  lime,  and  after  slightly  moistening  with  water,  the 
mixture  is  moulded  into  bricks  under  strong  pressure  (about 
60  tons  per  square  inch).  From  the  press  the  bricks  are  at 
once  placed  in  large  steel  cylinders  (called  autoclaves)  about 
7  ft.  diameter  and  generally  60  to  80  ft.  long,  where  high 
pressure  steam  (100  to  150  Ib.  per  square  inch)  acts  on  them 
for  10  to  15  hours  (in  America  7  to  12  hours) .  Chemical  action 
results  in  the  formation  of  a  hydrated  calcium  silicate,  which 
acts  as  a  strong  bond  between  the  larger  sand  grains.  About 
15  per  cent,  of  the  sand  should  pass  through  zoo-mesh  sieve, 
in  order  to  be  able  to  combine  with  the  lime  to  form  the  bond. 
The  other  grains  should  be  of  assorted  sizes,  so  as  to  give  the 
greatest  density,  but  should  not  include  pebbles  larger  than 
half-inch.  The  sand-lime  brick,  which  is  usually  light-grey 
or  nearly  white,  is  much  stronger  than  an  ordinary  mortar 
brick.  It  may  be  stained  by  adding  suitable  colouring 
oxides  or  pigments  which  are  not  affected  by  lime,  etc. 

FIREBRICKS. 

The  term  "firebrick"  is  employed  to  distinguish  such 
bricks  as  can  be  used  in  fire-places,  furnace  linings,  flues,  etc., 
where  ordinary  bricks  would  be  liable  to  melt.  Sometimes  the 


CERAMIC  INDUSTRIES  257 

term  is  made  to  include  also  bricks  made  of  silica,  magnesia, 
etc.,  but  in  a  more  restricted  sense  firebricks  are  understood 
to  be  made  of  fireclay  or  other  refractory  clays,  such  clays 
being  rich  in  alumina  and  silica,  but  low  in  ferrous  oxide, 
lime,  and  alkalies.  The  clay  is  never  used  alone — being 
liable  to  excessive  contraction  and  consequent  cracking — 
but  is  mixed  with  grog  or  with  sand,  coke,  graphite,  etc. 
Apart  from  the  direct  fusion  effects  of  temperature,  firebricks 
have  to  withstand  other  destructive  influences,  especially 
the  action  of  fluxing  materials  such  as  flue-dust,  slags, 
alkaline  vapours,  melting  alkalies,  and  certain  metallic 
oxides ;  they  must  also  be  able  to  resist  great  temperature 
changes,  and  be  strong  enough  to  bear  considerable  pressure 
at  temperatures  which  would  soften  ordinary  bricks.  Beyond 
the  difference  in  raw  materials,  there  is  no  essential  point 
of  difference  in  the  manufacture  of  firebricks  from  that  of 
ordinary  bricks,  except  that  more  care  is  used,  the  material 
is  stiff er,  and  the  firing  takes  place  at  a  higher  temperature. 
At  temperatures  not  exceeding  1000°  or  1100°  C.,  finely 
divided  silica  (only)  tends  to  combine  with  bases  and  thus 
promotes  fusibility.  At  higher  temperatures  the  coarser- 
grained  crystalline  silica  acts  similarly.  Excess  of  silica, 
beyond  what  can  combine  with  the  ordinary  bases  and 
alumina,  acts  as  a  refractory  constituent. 

In  the  absence  of  bases  or  other  fluxes  fusibility  increases 
with  increase  in  the  proportion  of  silica  to  alumina,  until  the 
composition  nearly  corresponds  to  A12O3,  i6SiO2,  after  which 
further  increase  in  the  proportion  of  silica  increases  the 
refractoriness.  Grog  acts  as  a  refractory  at  temperatures 
below  vitrification,  but  from  that  point  onwards  it  no  longer 
affects  the  fusibility.  The  presence  of  grog,  fairly  coarse- 
grained (according  to  the  thickness  of  the  articles)  is  par- 
ticularly advantageous  for  resisting  rapid  changes  of  tempera- 
ture, and  with  very  plastic  clays  the  amount  may  be  as  much 
as  2  parts  by  weight  of  grog  to  i  of  clay  ;  part  of  the 
grog  may  be  fine-grained,  as  the  fine  grains  can  get  between 
the  larger  grains,  and  so  allow  a  larger  proportion  of  grog  to 
be  used.  For  resisting  chemical  action,  as  of  fluxes  and 
c.  17 


258  SILICA   AND  THE  SILICATES 

vapours,  it  is  better  to  have  fine-grained  material,  and  to 
fire  to  incipient  vitrification. 

A  strongly-marked  feature  of  fireclay  bricks  when  heated 
under  load  is  that  they  gradually  soften  with  rise  of  tempera- 
ture until  the  brick  finally  collapses.  This  behaviour 
characterizes  aluminous  (fireclay)  and  zirconia  bricks. 

CRUCIBLES. 

Crucibles  are  required  to  withstand  very  high  tempera- 
tures, and  sometimes  other  severe  conditions  as  well.  Good 
crucibles  can  be  cooled  rapidly  without  cracking,  and  can 
also  resist  the  action  of  various  fluxing  materials,  as  in  the 
smelting  of  metals  or  of  glass. 

Hessian  Crucibles  are  made  of  fireclay  (composed  of 
about  71  per  cent,  silica,  25  alumina,  and  4  iron  oxide), 
mixed  with  one-third  to  half  its  weight  of  quartz  sand. 
Though  refractory,  and  capable  of  resisting  temperature 
changes,  their  porosity  and  coarseness  of  grain  renders  them 
unsuitable  for  some  chemical  operations,  being  corroded  by 
lead  oxide,  alkalies,  etc. 

Graphite  (or  Plumbago)  Crucibles  are  made  of  graphite 
mixed  with  20  or  25  per  cent,  of  refractory  clay  (usually 
fireclay).  They  are  highly  refractory,  and  have  high 
conductivity  for  heat. 

English  Crucibles  are  made  from  Stourbridge  clay  mixed 
with  50  per  cent,  of  coke.  Such  crucibles  reduce  metallic 
oxides  heated  in  contact  with  them. 

Crucibles  have  also  been  made  of  lime,  chalk,  magnesia, 
bauxite,  mixtures  of  bauxite  or  cryolite  and  magnesia,  of 
zirconia,  carborundum,  etc. 

FIRECLAY  WARE. 

Sanitary  and  other  articles  have  been  largely  made  of 
fireclays  both  in  this  country  and  abroad.  They  were 
formerly  subjected  to  a  biscuit  firing,  and  afterwards  covered 
with  a  lead  glaze.  In  some  cases  salt-glazing  was  resorted 
to  for  such  goods. 


CERAMIC  INDUSTRIES  259 

Such  ware  is  now  commonly  subjected  to  only  a  single 
firing  in  large  rectangular  muffle  kilns  with  arched  roofs. 
The  articles — baths,  lavatories,  sinks,  etc. —  are  first  made  of 
a  stiff,  slightly  moist  mixture  of  clay  and  grog,  the  latter 
being  either  the  same  kind  of  clay  calcined,  or  clean  refractory 
fireclay  bricks,  etc.,  which  are  crushed  and  sieved  to  give 
various  grades.  In  many  cases,  especially  for  the  larger 
pieces,  a  proportion  of  saw-dust  or  similar  carbonaceous 
material  is  added,  which  burns  off  in  the  firing  and  gives  a 
more  porous  product. 

Fireclay  goods  are  often  finished  white  (instead  of  buff), 
which  is  effected  by  covering  the  finished  clay  surface  with 
a  white  clay  "  body  "  or  "  engobe/'  and  covering  that  in 
turn  with  a  transparent  leadless  porcelain  glaze,  or  the 
engobe  is  sometimes  dispensed  with,  and  the  clay  covered 
with  a  similar  glaze  rendered  opaque  and  white  by  addition 
of  suitable  material  (as  tin  oxide) ,  the  composition  being  also 
modified  in  other  respects  if  necessary  or  desirable.  Both 
engobe  and  glaze  are  generally  applied  by  preparing  the 
sloppy  material  with  water  and  gelatine  (or  similar  sub- 
stance) and  brushing  it  on  the  ware,  as  many  coats  as  may  be 
considered  necessary  being  applied,  with  drying  between 
two  consecutive  coats.  The  smaller  articles  are  commonly 
glazed  by  dipping.  The  firing  is  usually  carried  out  at  a 
comparatively  high  temperature,  the  whole  period  of  firing 
being  rarely  less  than  12  or  14  days,  and  sometimes  longer. 


BRICKS. 

These,  as  the  name  implies,  are  made  of  silicious  material 
occurring  in  the  form  of  silica  rock  (quartzite,  ganister, 
flint,  etc.),  or  of  loose  natural  material  of  similar  character. 

The  chief  points  to  be  considered  are  mechanical  strength, 
melting  point,  and  behaviour  on  heating,  especially  under 
load.  From  experience  it  is  found  that  for  producing  strong 
bricks  the  quartzite  should  be  hard,  and  when  crushed  should 
not  give  rounded  grains  but  angular  fragments  of  different 
sizes  and  shapes.  The  rock  should  contain  96  to  98  per  cent. 


26o  SILICA   AND   THE  SILICATES 

silica,  with  alumina  and  iron  oxide  together  preferably  not 
exceeding  2*5  per  cent,  (usually  about  1*75  is  present),  and 
less  than  0*5  per  cent,  of  alkalies.  It  is  remarkable  that 
pure  quartzite  containing  as  much  as  99  per  cent,  silica  has 
not  proved  satisfactory  for  making  silica  bricks. 

Quartzite  suitable  for  refractory  purposes  does  not  crack 
when  strongly  heated,  but  shows  considerable  expansion 
after  the  first  firing,  and  not  much  further  change  after 
subsequent  firings.  Unsuitable  quartzites — especially 
coarsely  crystalline  varieties — crack  badly  when  heated,  and 
usually  expand  slightly  when  first  fired,  but  considerably 
in  subsequent  firings. 

Silica  bricks  are  made  from  quartzite  (or  similar  silicious 
material)  with  usually  lime  or  clay  as  bond.  I^ess  than 
1*5  per  cent,  lime  gives  weak  bricks,  and  with  more  than 
3  per  cent,  the  refractoriness  is  decreased  ;  about  1*75  to 
2  per  cent,  gives  the  best  results.  When  fired  for  the  first 
time,  the  quartz  of  silica  bricks  is  largely  converted  into 
cristobalite,  the  remainder  being  similarly  transformed  very 
slowly  in  subsequent  firings.  The  cristobalite  in  its  turn  is 
inverted  to  tridymite,  slowly  at  first,  but  afterwards  more 
rapidly,  the  actual  rate  in  all  cases  depending  on  the  firing 
temperature  and  the  time  ;  variations  in  grain  size  have 
comparatively  slight  effect  at  very  high  temperatures  (cone 
13  and  upwards).  All  the  lime  passes  into  the  ground  mass 
which  cements  the  silica  crystals,  and  accordingly  it  becomes 
more  concentrated  in  coarsely-ground  material  (which  has 
the  smallest  proportion  of  fine  particles).  An  increase  in  the 
amount  of  lime  should  hasten  the  inversion  of  cristobalite 
to  tridymite,  the  latter  forming  a  strong  framework  of 
crystals.  At  1250°  C.  the  specific  volumes  of  cristobalite 
and  tridymite  are  about  14  to  15  per  cent,  greater  than  that 
of  quartz.  Tridymite  expands  much  less  than  cristobalite 
on  heating.  The  gradual  inversion  of  cristobalite  into 
tridymite  probably  takes  place  in  furnace  linings.  The 
sp ailing  or  cracking  of  silica  bricks  under  quick  temperature 
changes  is  probably  owing  mainly  to  the  rapid  expansion 
(about  3  per  cent.)  which  accompanies  the  inversion  from 


CERAMIC  INDUSTRIES  261 

a  to  j8  cristobalite  between  230°  and  270°  C.  (The  a-f$  quartz 
inversion  at  575°  C.  gives  a  volume  change  of  about  1*4  per 
cent.)  This  sp ailing  tendency  of  silica  bricks  diminishes 
with  repeated  firings. 

A  large  proportion  of  tridymite  is  apparently  advan- 
tageous in  silica  bricks — at  any  rate  if  not  heated  continu- 
ously above  1470°  C. — because  tridymite  shows  less  tem- 
porary expansion  than  either  cristobalite  or  quartz,  because 
it  has  a  greater  specific  volume  and  shows  no  permanent 
expansion  after  repeated  firings  below  its  melting  point,  and 
because  tridymite  bricks  would  bear  rapid  temperature 
changes  with  slight  risk  of  cracking. 

At  temperatures  continuously  above  1470°,  a  tridymite 
brick  would  be  likely  to  revert  slowly  to  cristobalite  ; 
tridymite  is  not  known  to  be  formed  under  any  conditions 
above  1470°  C.  I^ong-continued  firing  would  favour  the 
formation  of  tridymite  more  than  repeated  firings.  Hence 
in  firing  silica  bricks,  the  highest  temperature  should  be  held 
for  a  considerable  time  ;  the  rate  of  inversion  is  more  rapid 
at  cone  14  or  15  than  at  cone  13. 

Bleininger  and  Ross,  on  heating  the  prepared  mixture 
to  1300°  C.,  observed  no  tridymite  and  not  more  than  5  per 
cent,  cristobalite.  At  1350°  the  cristobalite  was  57*13  per 
cent.,  the  rest  being  quartz  and  calcium  silicate.  At  1400°  C. 
there  was  72^17  per  cent,  cristobalite,  at  1450°  81*26  per  cent., 
and  at  1500°  C.  90  per  cent.,  with  a  little  tridymite  in  each 
case.  The  minimum  firing  temperature  of  silica  bricks 
should  apparently  be  cone  18.  Seaver  found  at  still  higher 
temperatures  (up  to  a  maximum  of  1630°  C.)  no  clear 
indication  of  the  formation  of  tridymite,  but  practically  all 
cristobalite. 

Rapid  crystallization  of  silica  from  a  melted  glass  always 
gives  cristobalite  in  the  first  instance,  and  this  crystallization 
serves  to  unite  the  undissolved  silica  grains.  Only  the  very 
fine  grains  in  silica  bricks  are  dissolved,  along  with  a  little  of 
the  outside  of  the  larger  grains,  the  latter  being  eventually 
transformed  into  cristobalite  with  swelling  of  the  grains. 
Formation  of  cristobalite  by  solution  in  the  fluxes  involves 


262  SILICA   AND   THE  SILICATES 

no  swelling,  the  crystals  being  deposited  in  the  spaces  left 
free. 

According  to  I^e  Chatelier,  cristobalite  is  less  stable  than 
tridymite  at  high  temperatures,  and  tends  to  become  trans- 
formed into  tridymite,  and  all  the  conditions  which  promote 
the  change  of  quartz  into  cristobalite,  afterwards  (if  pro- 
longed sufficiently)  promote  change  of  cristobalite  into 
tridymite.  In  steel  furnaces,  the  transformation  of  silica 
bricks  into  tridymite  becomes  complete,  at  least  where  the 
temperature  exceeds  1400°  C.  L,e  Chatelier  found  that  in 
well-fired  silica  bricks  there  is  a  continuous  network  of 
tridymite,  in  the  pores  of  which  the  melted  mass  lodges 
without  reducing  the  mechanical  resistance.  The  formation 
of  this  network  results  from  solution  of  the  quartz  in  the 
melted  magma,  and  recrystallization  of  the  silica  first  as 
cristobalite,  and  then  as  tridymite.  Underfired  bricks,  in 
which  no  network  is  formed,  consist  of  quartz  grains  floating 
in  the  melted  material ;  these  are  at  high  temperatures 
plastic  like  fireclay  or  magnesia  bricks.  The  most  favourable 
conditions  for  the  formation  of  the  tridymite  network  seem 
to  be  several  days  firing  at  a  temperature  approaching 
1450°  C. — below  that  at  which  the  quartz  is  directly  and 
rapidly  transformed  into  cristobalite — especially  when  the 
quartz  is  fine-grained,  though  a  certain  proportion  of  large 
grains  is  necessary  to  prevent  the  formation  of  cracks,  which 
develop  easily  in  uniformly  fine-grained  material.  If  a 
brick  containing  unchanged  quartz  grains  is  suddenly  heated 
to  a  temperature  at  which  quartz  is  rapidly  transformed,  the 
resulting  expansion  shatters  the  network  and  deprives  the 
brick  of  its  strength. 

D.  W.  Ross  found  that  on  re-heating  commercial 
(American)  silica  bricks  to  1400°,  1450°,  and  1500°  C. 
respectively,  there  was  usually  increased  porosity,  the  in- 
crease in  pore  space  being  roughly  proportional  to  the 
increase  in  volume  of  the  solid  material. 

Artificial  tridymite  and  cristobalite  have  specific  gravities 
of  about  2*27  and  2*33  respectively,  and  to  determine 
approximately  the  degree  of  firing  it  is  only  necessary  to 


CERAMIC  INDUSTRIES  263 

find  the  specific  gravity  of  a  piece  of  the  brick  from  the 
relationship:  sp.  gr.=  dry  weight  divided  by  volume  of 
the  solid  material,  i.e.  by  exterior  volume  (=  wet  weight  less 
suspended  weight)  minus  pore  volume  ( =  wet  weight  less 
dry  weight).  This  method  might  readily  be  used  for  rapid 
inspection  work,  and  is  independent  of  any  changes  in 
porosity.  The  average  specific  gravity  of  bricks  from  ten 
different  works  was  2 '3546  after  firing  once  at  1400°  C.,  and 
the  average  volume  expansion  was  1*45  per  cent.  The 
average  volume  expansion  of  two  series  of  14  each,  fired  once 
at  1450°  C.  and  1500°  C.  was  1*38  and  5*66  respectively. 
The  quality  of  silica  refractories  can  be  tested  by  comparing 
the  percentage  volume  increase  of  materials  which  have  been 
reheated  (once  or  oftener).  The  extreme  porosities  22*64 
and  31*96  per  cent,  (of  original  brick)  show  the  range  likely 
to  be  found  in  American  industrial  practice.  A  combination 
of  the  volume  and  specific  gravity  methods  should  determine 
accurately  whether  or  not  silica  refractories  have  been 
properly  fired. 

For  making  silica  bricks,  the  silica  rock  is  crushed,  then 
ground  on  pan-grinders  to  the  necessary  fineness,  usually  to 
pass  a  sieve  of  16  meshes  to  the  linear  inch,  the  bond  (mostly 
lime  or  fireclay)  being  added  to  the  silica  on  the  pan.  L,ime 
is  added  during  the  grinding,  as  milk  of  lime.  Part  of  the 
silica  is  sometimes  used  in  the  form  of  grog,  obtained  from 
old  or  defective  silica  bricks,  crushed  or  ground  to  suitable 
size.  In  some  cases  a  small  proportion  of  sand  is  added  as 
well  as  grog.  The  lime  helps  to  give  the  material  a  certain 
degree  of  plasticity.  L,e  Chatelier,  Philipon,  and  others  in 
France,  and  Mottram  in  England,  recommend  the  use  of  a 
proportion  (up  to  30  per  cent.)  of  very  finely  ground  silica. 
Mottram  specifies  grinding  15  to  30  per  cent,  of  the  raw 
silicious  material  along  with  the  lime,  clay,  or  other  binder, 
to  a  fine  slip,  and  then  adding  this  to  the  rest  of  the  graded 
silica  material. 

The  shaping  of  the  silica  bricks  may  be  done  in  open 
moulds  (made  of  wood  or  iron)  ;  the  finishing  is  effected 
either  by  using  *a  hand-press  or  a  power-driven  press  with  a 


264  SILICA   AND  THE  SILICATES 

revolving  table.  For  hand-making,  an  iron  plate  is  placed 
under  the  mould  and  the  mixture  is  worked  and  pressed  into 
shape  ;  the  mould  is  then  lifted  off,  and  the  brick  or  bricks 
with  the  supporting  plate  are  set  to  dry.  For  hand-made 
bricks  9  to  n  per  cent,  water  is  best  for  avoiding  moulding 
defects. 

The  dried  bricks  are  set  (as  freely  as  possible)  in  the  kiln, 
which  is  usually  a  circular  down-draught  or  up-draught  kiln. 
The  bricks  should  be  thoroughly  dried,  before  heating  up, 
if  fire  cracks  are  to  be  avoided.  The  firing  may  last  7  to  10 
days,  at  a  maximum  temperature  of  1400°  to  1600°  C.,  but 
sometimes  only  4  days.  In  the  United  States  they  are 
fired  10  to  15  days,  including  i  to  3  days  at  the  maximum 
temperature  (cone  16  or  higher).  They  are  allowed  to  cool 
slowly  down  to  700°  or  800°  C.,  Dinas  or  ganister  bricks  being 
very  liable  to  crack  if  cooled  suddenly  or  unevenly.  In  the 
United  States,  cooling  usually  takes  about  5  days.  For 
certain  special  purposes  silica  bricks  may  undergo  several 
firings  at  higher  temperatures. 

The  desirable  qualities  in  silica  bricks  with  lime  bond 
are  resistance  to  temperature  changes  and  to  the  highest 
temperature  in  the  furnace,  resistance  to  combustion  and 
reaction  products  in  the  atmosphere  of  the  furnace,  and 
regular  but  not  excessive  expansion.  Texture  is  also  very 
important. 

In  contrast  with  fireclay  bricks,  silica  bricks  and  silicious 
bricks,  though  they  may  show  some  signs  of  a  gradual 
deformation  when  heated  under  load,  are  characterized  by  a 
more  or  less  sudden  collapse.  In  a  less  degree  the  same  is 
typical  also  of  magnesia  and  chrome  bricks. 

In  a  seasoned  silica  brick,  the  darker  and  denser  portion 
nearest  the  inside  of  the  furnace  is  more  refractory  and 
resistant  to  temperature  changes  without  fracture.  Accord- 
ing to  C.  John,  a  magnet  applied  to  the  finely  crushed  dark 
portion  removes  15  per  cent,  magnetic  oxide  of  iron,  which  is 
the  stable  oxide  at  high  temperatures,  and  which  neither 
combines  with  silica  nor  forms  a  solid  solution  at  furnace 
temperatures.  It  is  only  with  an  oxidizing  atmosphere 


CERAMIC  INDUSTRIES  265 

that  magnetic  oxide  of  iron  is  advantageous  in  silica 
bricks. 

The  action  of  iron  oxide  on  silica  is  very  important.  In 
France,  Rengade,  and  also  I^e  Chatelier  and  Bogitch,  found 
in  silica  bricks  removed  from  steel  furnaces  several  distinct 
zones  :  (i)  a  more  or  less  completely  fused  grey  zone,  in 
which  the  cristobalite  has  been  partly  and  irregularly  inverted 
to  tridymite ;  (2)  an  apparently  homogeneous  blackish- 
brown  zone,  composed  of  large  crystals  of  tridymite  scattered 
regularly  in  a  black  ferruginous  flux  ;  (3)  a  zone  strewed 
with  white  specks  formed  by  the  largest  fragments  of 
quartzose  rock,  which  fragments  retain  their  shape,  though 
the  quartz  has  been  inverted  to  very  fine  tridymite  ;  and 
(4)  a  zone  consisting  of  cristobalite  grains  surrounded  by 
very  fine  tridymite.  From  analyses  it  appeared  that  the 
grey  zone — in  contact  with  the  flame,  and  exposed  to  the 
action  of  ferruginous  dust — contains  less  of  the  bases  (iron 
oxide  and  lime)  than  the  upper  layers,  and  the  brown  and 
clear  yellow  zones  contained  more  lime.  By  experimental 
investigation,  I,e  Chatelier  and  Bogitch  found  that  the  iron 
oxide  penetrated  inwards  by  capillary  ascent,  and  the 
silicates  of  iron,  thus  rising,  push  before  them  the  alumino- 
silicates  of  lime  pre-existing  in  the  brick.  In  case  an  acci- 
dental flash  should  melt  the  superficial  layer,  the  next  layer, 
suddenly  brought  into  contact  with  the  flame  while  still 
rich  in  bases,  will  also  melt,  and  thus  a  brick  can  lose  half 
its  height  in  a  few  minutes ;  this  is  one  reason  why  the 
management  of  a  steel  furnace  is  such  a  delicate  operation. 

Graham  and  Stead,  in  this  country,  separately  made 
independent  observations  which  essentially  agree  with  the 
foregoing,  as  also  did  Bigot  in  France.  An  interesting  result 
of  these  investigations,  as  pointed  out  by  Bigot,  is  that  parts 
of  the  silica  bricks  in  use — viz.,  the  grey  parts— though  con- 
taining only  85  per  cent,  silica,  with  15  per  cent,  bases, 
proved  to  be  as  refractory  as  products  containing  95  per 
cent,  silica,  and  are  much  superior  as  regards  crushing 
strength  at  all  temperatures — though  it  is  commonly 
assumed  that  silica  should  not  be  less  than  94  per  cent. 


266  SILICA   AND   THE  SILICATES 

Also  the  same  grey  parts  contain  10  to  14  per  cent,  lime  and 
iron  oxide  together,  though  3  to  4  per  cent,  is  usually  re- 
garded as  the  permissible  limit.  Bigot  also  states  that 
enamelled  silica  bricks  from  the  frieze  of  archers  at  the 
palace  of  Darius,  made  500  years  before  the  Christian  era, 
which  are  now  in  the  lyouvre  Museum,  contain  about  n  per 
cent,  of  foreign  matters  (other  than  silica),  and  are  equally 
refractory.  Another  significant  statement  by  Bigot  is  that 
ferruginous  sandstones,  containing  from  3  to  8  per  cent, 
iron  oxide  are  suitable  for  making  silica  products. 

It  is  worthy  of  note  that  the  Dinas  firebricks,  the  first 
silica  bricks  ever  made  for  use  as  refractories,  were  the 
invention  of  W.  W.  Young,  a  Glamorganshire  land-surveyor, 
who  in  1822  established  a  company  to  make  the  bricks. 
The  material  occurs  in  the  Vale  of  Neath  in  the  state  of  solid 
rock,  and  also  disintegrated  like  sand.  Its  colour  when  dry 
is  pale  grey.  The  method  of  manufacture  was  essentially 
like  the  method  of  making  silica  bricks  at  the  present  time. 

Some  other  refractory  materials  may  be  briefly  noticed, 
though  they  are  mostly  non-silicious  : — 

Magnesite,  or  native  magnesium  carbonate,  is  converted 
into  magnesia  on  calcining.  The  amorphous  powder  so 
obtained  has  a  specific  gravity  of  about  3*2  to  3*3,  but  it  may 
be  considerably  higher  if  calcined  at  a  high  temperature  for  a 
prolonged  period.  If  strongly  calcined  so  as  to  become 
crystalline,  the  specific  gravity  is  nearly  37.  The  specific 
gravity  of  the  mineral  periclase  (crystalline  magnesia)  varies 
from  3-50  to  375.  The  change  in  specific  gravity  is  accom- 
panied by  a  corresponding  change  of  volume,  and  such 
change  is  liable  to  cause  sp ailing  (cracking  or  flaking  off)  at 
high  temperatures.  The  most  extensive  deposits  of  magne- 
site  occur  in  Greece,  Hungary,  and  Styria  (Austria),  but  it 
is  also  found  in  Italy,  India,  the  United  States,  and  Canada. 
The  magnesite  from  Greece  is  the  purest,  but  the  Austrian 
has  been  found  best  for  making  bricks,  owing  to  its  containing 
about  5  to  7  per  cent,  iron  oxide. 

The  magnesite  has  to  be  calcined  before  making  into 
bricks,  etc.  The  magnesium  carbonate  is  decomposed  at 


CERAMIC  INDUSTRIES  267 

400°  C.,  but  a  much  higher  temperature  is  necessary  to 
ensure  that  all  the  CO2  is  removed. 

The  calcined  magnesite  (really  magnesia)  is  crushed  and 
ground  (to  about  T^  in.  grains)  much  in  the  same  way  as  silica. 
The  unfired  magnesia  bricks  (often  called  magnesite  bricks) 
crumble  easily,  being  almost  devoid  of  tenacity.  The  dried 
bricks  are  set  in  chamber  kilns  or  in  up-draught  beehive 
kilns.  Strong  and  dense  bricks  are  required  for  steel 
furnaces,  and  to  obtain  them  a  very  high  firing  temperature 
is  necessary.  The  temperature  must  be  raised  very  gradu- 
ally in  the  early  stages,  and  a  good  soaking  should  be  given 
at  the  maximum  temperature.  English  bricks  are  hand- 
pressed,  but  hydraulic  pressing  would  give  better  products. 

Magnesia  bricks  do  not  soften  gradually,  like  fireclay 
bricks,  when  heated  under  load.  They  are  chiefly  used  for 
linings  of  basic  steel  furnaces  (both  converters  and  open- 
hearth  furnaces)  and  electric  furnaces.  The  calcined 
magnesite  is  also  used  for  making  repairs. 

Dolomite  has  properties  somewhat  similar  to  those  of 
magnesite  ;  it  is  equally  refractory,  but  the  presence  of  a 
large  percentage  of  lime  makes  it  much  inferior  in  other 
respects,  mainly  because  it  readily  slakes  in  air.  It  cannot 
be  kept  long  after  calcination  without  being  injuriously 
affected  owing  to  slaking  of  the  lime  present,  especially  if  the 
calcination  has  not  taken  place  at  a  very  high  temperature. 
Dolomite  is  largely  used  for  temporary  repairs  of  hearths  in 
basic  furnaces,  because  it  sets  more  quickly  than  calcined 
magnesite,  besides  being  much  cheaper,  but  it  does  not  set 
as  solidly  as  the  magnesite. 

Better  service  is  given  in  the  furnace  by  special  dolomitic 
refractories,  in  which  the  dolomite  is  treated  with  pulverized 
basic  slag,  iron  ore,  etc. 

Chrome  or  Chromite  Bricks  are  used  in  parts  of  furnaces 
where  a  neutral  refractory  is  required  without  the  reducing 
action  of  carbon  refractories.  They  are  sometimes  used  as 
a  buffer  between  the  magnesia  bricks  at  the  bottom  of 
a  steel  furnace,  and  sometimes  as  a  neutral  layer  between 
the  basic  (magnesia)  bricks  below  and  the  acid  (silica)  bricks 


268  SILICA   AND   THE  SILICATES 

above  in  basic  furnaces.     They  are  also  used  for  certain 
special  purposes. 

Chrome  bricks  contain  up  to  about  40  per  cent,  chromic 
oxide.  The  method  of  manufacture  is  generally  similar  to 
the  method  used  for  magnesia  bricks,  no  bond  being  used 
as  a  rule. 

Alumina  is  used  chiefly  as  a  product  from  bauxite. 
Sometimes  raw  bauxite  is  employed,  generally  mixed  with 
fireclay  for  bonding  purposes,  but  has  the  great  disadvantage 
of  excessive  contraction.  Fused  bauxite  has  been  brought 
into  use  in  two  forms  :  alundum,  for  which  the  bauxite  has 
been  fused  in  an  electric  furnace,  and  corindite,  in  the  making 
of  which  the  bauxite  is  heated  with  coke  or  other  carbonaceous 
substance  (according  to  N.  I^ecesne's  patent),  forming  in  the 
first  instance  aluminium  carbide,  the  subsequent  combustion 
of  which  melts  the  re-formed  alumina  and  even  volatilizes 
silica.  Both  alundum  and  corindite  are  high-grade  refrac- 
tories, but  the  use  of  the  former  is  limited  by  reason  of  its 
high  cost,  and  the  latter  has  not  yet  been  produced  on  any 
large  scale. 

Sillimanite  (Al2SiO5=Al2O3.SiO2)  has  valuable  pro- 
perties as  a  refractory,  especially  when  fused.  It  does  not 
occur  naturally  in  quantity,  but  A.  Malinovszky  has  recently 
prepared  fused  artificial  sillimanite  from  a  mixture  of 
aluminous  materials  and  coke  by  a  method  very  similar  to 
lyecesne's  process  for  making  fused  bauxite  (corindite)  and 
this  fused  artificial  sillimanite  so  obtained  is  said  to  have 
no  expansion  or  contraction  at  any  temperature. 

Zirconia  is  another  high-grade  refractory  which  has 
sprung  into  a  prominent  position  within  the  last  few  years. 
Formerly  regarded  as  a  comparatively  rare  material,  it  ha* 
been  discovered  in  considerable  quantity  in  Brazil,  and  is 
now  an  important  export  from  that  country. 

Zirconia  possesses  a  most  exceptional  combination  of 
properties :  it  is  highly  refractory  (the  melting  point  for  the 
ordinary  raw  material  being  little  short  of  2000°  C.) ;  it  has 
very  low  conductivity  for  heat,  and  very  low  coefficient 
of  thermal  expansion  (the  latter  being  nearly  the  same  as 


CERAMIC  INDUSTRIES  269 

that  of  quartz  glass),  so  that  it  can  be  plunged  while  red-hot 
into  water  without  cracking  or  breaking  ;  it  is  highly 
resistant  to  acid  and  basic  slags.  All  these  properties  are 
possessed  by  native  zirconia  containing  80  per  cent,  or  more 
ZrO2.  It  also  resists  fused  cyanides  and  alkalies,  the  only 
substances  which  attack  it  seriously  being  fused  bisulphates 
and  fluorides.  Its  low  conductivity  for  electricity  (except 
when  strongly  heated)  makes  it  well  adapted  for  insulating 
purposes,  as  in  electrical  heating  apparatus.  Other  impor- 
tant applications  are  possible  from  the  fact  that  though 
very  hard  it  is  very  voluminous. 

At  high  temperatures  in  contact  with  carbon,  zirconia  is 
very  apt  to  form  zirconium  carbide,  which,  though  very 
hard  and  refractory,  has  not  the  other  valuable  properties  of 
zirconia.  This  will  not  happen  in  an  ordinary  furnace,  where 
there  is  a  sufficiently  oxidizing  atmosphere. 

Zirconia  has  been  used  for  coating  (or  replacing)  the  lime 
and  magnesia  pencils  used  in  the  Drummond  (or  lime)  light, 
especially  in  spectroscopy  and  microphotography  ;  in  the 
Bleriot  lamps  forming  headlights  for  automobiles,  a  rod  of 
zirconia  is  heated  in  a  blow-pipe  flame  fed  with  oil  vapour 
and  oxygen  ;  zirconia  is  also  used  in  the  filaments  of  the 
Nernst,  Sanders,  and  Zernig  electric  lamps,  a  preheating  of  the 
filament  being  necessary  to  enable  it  to  conduct  the  current. 

When  more  readily  obtainable  at  moderate  prices,  native 
zirconia  will  be  very  useful  as  a  lining  for  electric  arc  furnaces, 
though  the  tendency  to  form  zirconium  carbide  should  not 
be  lost  sight  of. 

According  to  Podzsus,  zirconia  which  has  been  fused  or 
heated  to  beyond  2000°  C.  is  well  suited  for  making  re- 
fractory ware,  having  no  tendency  to  crack  like  raw  zirconia 
used  without  a  bond.  The  fused  zirconia  is  ground  finely, 
part  of  it  so  very  finely  that  it  assumes  a  colloid  form  and 
gives  sufficient  plasticity  to  the  whole. 

About  i  per  cent,  of  alumina,  thoria,  or  yttria  forms  a 
serviceable  addition  to  zirconia  for  some  purposes. 

Zirconia  makes  a  non-poisonous,  non-discolouring,  per- 
manent white  paint  of  good  covering  power,  not  affected  by 


270  SILICA   AND   THE  SILICATES 

sulphuretted  hydrogen,  acids,  or  alkalies.     It  has  also  been 
used  in  making  X-ray  pictures,  and  for  other  purposes. 

Zircon  is  the  native  zirconium  silicate  (ZrSiO4),  which  is 
of  widespread  occurrence,  but  not  very  often  in  large 
supplies.  It  is  useful  as  a  refractory,  though  not  comparable 
with  zirconia  itself  in  that  respect.  It  has  been  found 
suitable  for  spark  plugs  and  high-tension  insulators,  and  also 
for  use  in  crucibles,  saggars,  etc. 


BRITISH  DEVELOPMENTS. 

Immediately  after  the  outbreak  of  war  steps  were  taken 
to  ensure  a  supply  of  Seger  cones,  previously  obtained  from 
Germany,  and  used  in  many  works  for  the  control  of  firing 
operations.  Dr.  J.  W.  Mellor  undertook  to  supervise  the 
manufacture  of  them  at  the  Stoke- on-Trent  Pottery  School, 
and  the  cones  have  been  supplied  regularly  since  as  required, 
without  a  break. 

The  same  school  has  been  for  some  years  well  equipped 
for  carrying  out,  not  only  the  work  of  teaching  and  training, 
but  also  original  investigation,  and  its  capacity  in  the  latter 
direction  has  been  greatly  extended.  In  1915,  the  Governors 
of  the  School,  in  co-operation  with  local  manufacturers, 
asked  the  Department  of  Scientific  and  Industrial  Research 
to  assist  them  to  investigate  the  methods  of  making  hard 
porcelain  similar  to  the  continental  domestic  ware.  It  was 
eventually  arranged  that  Dr.  Mellor  and  Mr.  Bernard  Moore 
should  undertake  the  inquiry,  the  department  making  a 
substantial  contribution  towards  the  cost.  A  small  factory 
outfit  was  provided,  with  a  suitable  gas-fired  kiln.  Mean- 
while, preliminary  laboratory  experiments  had  been  made,  and 
early  in  1916  operations  were  commenced  on  a  larger  scale. 
It  was  intended  from  the  first  to  use  materials  of  British 
origin,  and  English  methods  of  working  (with  minimum 
modification)  and  of  firing  were  to  be  adhered  to ;  other 
points  borne  in  mind  were  the  colours  and  methods  of 
decoration,  and  the  cost  of  manufacture  (which  should  not 
exceed  that  of  the  ware  with  which  it  would  have  to 


CERAMIC  INDUSTRIES  271 

compete.  Within  eighteen  months  of  the  beginning  of  experi- 
ments, marketable  ware  was  produced.  A  number  of  firms 
are  now  preparing  to  produce  the  new  ware  commercially 
when  the  present  great  demand  for  ordinary  classes  of  ware 
eases  off  somewhat. 

Through  the  enterprise  of  several  firms  in  different 
parts  of  England  (Stoke-on-Trent,  Condon,  Worcester),  the 
threatened  stoppage  of  supplies  of  chemical  laboratory  ware 
was  met  by  the  production  of  new  ware  of  the  type  required. 

STATISTICS. 

The  following  figures  showing  the  weights  and  values  of 
the  total  exports  and  imports,  between  this  country  on  the 
one  hand,  and  foreign  countries  and  British  possessions 
respectively  on  the  other  hand,  are  significant.  They  include 
china  ware  and  parian,  tiles  (except  for  roofing  and  street, 
paving),  sanitary  ware,  other  earthenware,  electrical  ware, 
door  fittings,  chemical  ware,  jet,  rockingham,  glazed  terra- 
cotta ware,  stoneware,  red  pottery,  brown  and  yellow  ware. 
Had  there  been  space  available  for  giving  details,  they  would 
also  prove  interesting. 

Exports  to  Foreign  Countries.  Exports  to  British  Possessions. 

Year.               Cwts.                      £  Cwts.                      £ 

1913  2416636    1946355  1625451     I453IOI 

1914  1883246    1418992  1282549    1177860 

1915  1433874    1048071  1026936    1006369 

1916  1138154    1303532  980520    1310764 

1917  942767   1542687       78l739   1164451 

1918  770099   1807534       573677   IH7332 

Imports  from 

Imports  from  Foreign  Countries.  British  Possessions. 

Year.  Cwts.  £  Cwts.  £ 

1913  831133     1089329         1646     7518 

1914  618428      748917         1195     6269 

1915  150057      194353          7§6     2946 

1916  87144      160300          552     2368 

1917  15890     31867        ii      37 

1918  4820      21451         4      17 


272  SILICA   AND   THE  SILICATES 

The  exports  of  clay  tobacco-pipes  were  as  follows  : — 

Exports  to  Foreign  Countries.  Exports  to  British  Possessions. 
Year.              Gross.                ^  Gross  £ 

1913  52268     4400         80567     8869 

1914  74264     5660         62776     6500 

1915  38816    3158       73338    7534 

1916  40231    4040       97471    13402 

1917  42439    6715       98951    17445 

1918  25719     5386         98781     20102 


LITERATURE. 

"  English  Earthenware  and  Stoneware,"  W.  Burton.     London,  1904. 

'  Porcelain,"  W.  Burton.     London,  1906. 

'  English  Earthenware,"  A.  H.  Church.     London,  1904. 

'  English  Porcelain,"  A.  H.  Church.     London,  1904. 

'  Gesammelte  Schriften,"  H.  A.  Seger.     Berlin,  1896. 

'  Collected  Writings  of  H.  A.  Seger  "  (American  Translation).  Easton, 
Pa.,  1902. 

"  Traite  des  Industries  Ceramiques,"  E.  Bourry.     Paris,  1897. 

"  Ceramic  Industries "  (English  Translation),  E.  Bourry.  London, 
1901  and  1911. 

"  Notes  on  the  Manufacture  of  Earthenware,"  E.  A.  Sandeman.  London, 
1901  and  1917. 

"  Chemistry  of  Pottery,"  K.  Langenbeck.     Easton,  Pa.,  1895. 

"  Chemistry  of  Pottery,"  S.  Shaw.     London,  1837  and  1900. 

"  A  Text-book  on  Ceramic  Calculations,"  W.  Jackson.     London,  1904. 

"  La  Ceramique  Indiistrielle,"  A.  Granger.     Paris,  1905. 

"  Manuel  de  Ceramique  Industrielle,"  Arnaud  et  Franche.     Paris,  1906. 

"  Ceramic  Chemistry,"  H.  H.  Stephenson.     London,  1912. 

"  Ceramic  Technology."     Edited  by  C.  F.  Binns.     London. 

"  Manual  of  Practical  Potting."     Edited  by  C.  F.  Binns.     London. 

"  Handbuch  der  Gesammten  Zonwaren  Industrie,"  B.  Kerl.     1907. 

"Trait6  des  Arts  Ceramiques,"  A.  Brongniart.     Paris,  1844  and  1877. 

' '  Report  on  the  Pottery  Industry. ' '  Washington,  1915.  This  embodies 
interesting  and  useful  statistics  relating  to  the  States,  the  United  Kingdom, 
and  various  continental  countries,  collected  by  the  U.S.  Dept.  of  Commerce. 

Thorpe's  "  Dictionary  of  Applied  Chemistry."  Revised  edition 
Especially  article  on  "  Pottery  and  Porcelain/'  by  William  Burton. 

Encyclopedia  Britannica,  nth  edition.  Articles  on  "  Ceramics,"  etc., 
by  William  Burton  and  others. 

Also  various  periodicals,  etc.,  which  will  be  more  fully  referred  to  at  the 
end,  but  especially  Transactions  of  the  Ceramic  Society  and  Transactions 
and  Journal  of  the  American  Ceramic  Society. 


SECTION  V.— GLASS  AND  ENAMELS 

GLASS-MAKING  AN  ANCIENT  INDUSTRY. 

THAT  glass  was  known  in  very  early  times  is  beyond  doubt, 
but  the  precise  period  of  its  invention  is  very  uncertain. 
According  to  a  tradition  mentioned  by  Pliny,  it  was  acci- 
dentally discovered  by  Phoenicians ;  some  mariners  who 
cooked  their  food  on  the  sands  at  the  mouth  of  the  river 
Belus  (a  small  stream  running  from  the  foot  of  Mount  Carmel 
in  southern  Phoenicia),  where  the  herbaceous  plant  known 
as  saltwort  or  glasswort  (Salsola  Kali)  is  common,  noticed 
that  when  the  sand  was  intimately  mixed  with  the  ashes 
of  the  plant,  it  melted  to  form  a  vitreous  substance.  Theo- 
phrastus,  about  300  B.C.,  refers  to  the  use  of  sand  from  the 
river  Belus,  and  from  that  time  glass  was  fairly  well  known 
in  most  countries.  Alexandria  (in  Egypt)  early  attained 
lasting  fame  for  the  skill  of  its  glass  workers,  who  supplied 
glassware  to  the  Romans  among  others.  From  remains  of 
furnaces  and  workshops,  it  is  known  that  glass  rods  and 
beads,  and  small  vases,  were  made  in  Egypt  as  early  as 
1400  B.C.  In  Britain  glass  was  obtained  by  trading  with  the 
Venetians,  long  before  it  was  made  in  this  country.  The 
earliest  authentic  record  of  glass  being  made  in  England  is 
in  1230  A.D.,  on  the  border  of  Sussex  and  vSurrey,  and  the 
manufacture  continued  only  there  until  the  latter  half  of  the 
sixteenth  century.  Glass  windows  were  little  used  here 
before  the  eleventh  century,  and  then  only  in  the  houses  of 
the  upper  classes  and  in  public  buildings,  whilst  in  ordinary 
private  houses  they  were  scarcely  used  until  the  thirteenth 
and  fourteenth  centuries ;  linen  cloths  or  wooden  lattices 
c.  273  i8 


274  SILICA   AND  THE  SILICATES 

were  the  means  previously  resorted  to  for  the  admission  of 
light. 


COMPOSITION  AND  PROPERTIES  OF  GLASS. 

The  components  of  glass  include  acid  oxides,  basic  oxides, 
and  certain  elements  and  compounds  which  occur  in  coloured 
glasses. 

The  acid  oxides  comprise  silica,  boric  oxide,  phosphoric 
anhydride,  and  arsenious  anhydride  (sometimes  possibly 
with  arsenic  anhydride). 

The  basic  oxides  fall  into  three  groups  :  those  of  the 
R2O  type,  including  Na2O,  K2O,  and  occasionally  L,i2O  and 
T12O  ;  those  of  the  RO  type,  including  CaO,  MgO,  BaO, 
ZnO,  MnO,  FeO,  PbO  ;  those  of  the  R2O3  group,  including 
A12O3  and  Fe2O3. 

The  additional  components  which  may  be  present  in 
coloured  glass  include  gold,  silver,  copper,  carbon,  sulphides 
of  sodium  and  calcium,  fluorides  of  calcium  and  aluminium, 
oxides  of  cobalt,  nickel,  chromium,  copper,  uranium,  tin. 

Silica  has  the  power  of  existing  in  the  vitreous  state,  that 
is,  the  amorphous  solid  condition,  and  the  only  other  in- 
organic binary  compounds  possessing  the  same  property 
are  boric  oxide  and  phosphoric  anhydride.  All  substances 
can  exist  in  the  amorphous  state,  but  then  they  are  nearly 
always  fluid  (liquid  or  gaseous).  The  normal  conditions  of 
existence  of  the  amorphous  state  always  correspond  to  the 
highest  temperature,  and  those  of  the  crystallized  state  to 
the  lowest,  whilst  the  temperature  of  fusion  or  of  crystalliza- 
tion is  a  transformation  point,  on  either  side  of  which  only 
one  of  the  two  varieties  is  stable  ;  even  by  such  a  device  as 
very  sudden  cooling  it  is  almost  impossible  to  preserve  a 
body  in  the  unstable  state  on  either  side  of  the  transformation 
point.  Silica  exhibits  the  peculiarity  of  being  easily  pre- 
served in  the  amorphous  state  at  all  temperatures  up  to  that 
at  which  it  becomes  completely  solid.  For  most  substances 
the  point  of  crystallization  corresponds  to  temperatures  at 
which  the  amorphous  state  of  the  same  substance  still 


GL.4SS  AND  ENAMELS  275 

exhibits  a  very  weak  viscosity,  the  mobility  of  the  particles 
then  making  crystallization  so  easy,  that  it  cannot  be 
prevented.  With  silica  the  point  of  crystallization  corre- 
sponds to  a  temperature  at  which  its  amorphous  condition 
is  fairly  viscous  or  even  pasty,  so  crystallization  is  so  slow 
that  it  can  be  quite  prevented  by  cooling  rather  slowly. 
Boric  oxide  and  phosphoric  anhydride  resemble  silica  in  this 
respect,  and  the  various  metallic  silicates,  borates,  and 
phosphates — unlike  other  metallic  salts — also  possess  the 
property  of  keeping  the  vitreous  state  more  or  less  easily. 
It  is  almost  impossible  to  crystallize  lead  silicate  (SiO2, 
PbO),  and  crystallization  of  the  corresponding  calcium  and 
magnesium  silicates  is  hindered  by  sudden  cooling.  Many 
definite  silicates,  found  well  crystallized  in  rocks,  give,  when 
melted,  glasses  which  cannot  be  crystallized  again  ;  ortho- 
clase  is  a  good  example,  and  it  is  owing  to  this  property  that 
it  can  be  used  for  making  ornamental  pearls. 

These  various  definite  compounds,  silica  and  silicates, 
can  be  mixed  in  the  amorphous  state,  and  thus  give  solid 
solutions  equally  amorphous,  which  essentially  constitute 
what  are  called  glasses.  These  solutions  have  all  the  pro- 
perties of  ordinary  liquid  solutions,  except  that  which  depends 
on  the  coefficient  of  viscosity.  All  glasses  are  at  ordinary 
temperatures  out  of  equilibrium,  the  stable  state  correspond- 
ing to  complete  crystallization.  Glasses  can  also  dissolve 
and  keep  in  the  amorphous  state  substances  which  alone 
would  not  take  that  state,  as  sodium  chloride  and  sulphate, 
magnesium  or  lithium  silicate,  etc.,  which  by  themselves 
would  crystallize  easily.  The  addition  of  bases  to  silica 
materially  reduces  the  viscosity  of  vitreous  silica.  The 
silica  crystallizes  much  more  easily  in  alkali  or  lead  glasses 
than  in  the  free  state. 

Many  glasses,  when  reheated  until  they  begin  to  become 
pasty,  are  able  to  crystallize  more  or  less  rapidly,  but 
generally  only  within  a  very  limited  range  of  temperature. 
This  confused  crystallization  of  glass — through  formation 
of  long  very  slender  crystals  in  a  pasty  mass — is  termed 
"  devitrification." 


276  SILICA   AND  THE  SILICATES 

The  passage  from  the  solid  to  the  liquid  state  of  an 
amorphous  substance  shows  no  sudden  fusing  point,  but 
according  to  the  composition  of  a  glass  the  coefficient  of 
viscosity  may  vary  more  or  less  rapidly,  and  this  forms  the 
basis  of  all  glass  manufacturing  processes. 

Grenet  (quoted  with  approval  by  I,e  Chatelier,  in  whose 
laboratory  Grenet  worked)  characterizes  the  range  of 
fusibility  of  glass  by  means  of  three  points :  (i)  The 
squatting  temperature  or  depression  temperature,  at  which  a 
vertical  glass  rod,  i  cm.  diameter  and  3  cms.  high,  becomes 
depressed  under  its  own  weight  to  about  half  its  height  (the 
load  supported  by  the  base  of  the  glass  being  about  10  g. 
per  sq.  cm.),  corresponding  to  the  normal  temperature  of 
glass-blowing,  to  the  solidification  temperature  of  ceramic 
glazes,  and  to  what  is  called  the  fusion  point  of  Seger  cones  ; 
(2)  the  annealing  temperature,  at  which  irregular  tensions 
(caused  by  too  rapid  cooling  of  glass)  disappear,  and  which 
is  notably  lower  than  the  squatting  temperature  industrially  ; 
it  is  the  temperature  at  which  glass  is  annealed  after  manu- 
facture ;  (3)  the  tempering  temperature,  at  which  the  glass 
begins  to  be  very  slightly  deformable  under  the  strongest 
force  it  can  support  without  breaking  ;  the  stress  in  these 
cases  may  be  estimated  at  1000  kg.  per  sq.  cm. 

Grenet's  experiments  gave  the  following  results  for 
St.  Gobain  glass  : — 

Squatting      Annealing     Tempering 
Composition.  Temp.  Temp.  Temp. 

2'6SiO2,  o'37Na2O,  o^CaO  800°  C.  604°  C.  524°  C. 
2'45Si02,  0'47Na20,  o^CaO  845°  C.  641°  C.  573°  C. 
2'45SiO2,  o'47Na2O,  o^PbO  720°  C.  460°  C.  390°  C. 

Replacing  the  lime  in  the  second  of  these  by  equivalent 
quantities  of  other  bases  gave  the  following  squatting 
points : — 

CaO    BaO    ZnO    PbO     K2O    U2O 

845°  1010°    890°    720°    705°     690° 

The  lowest  squatting  point  of  a  glass  studied  by  Grenet 


GLASS  AND  ENAMELS  277 

was  475°  C.  for  a  lead  borosilicate  of  the  composition 
o'5$iO2,  0'5B2O3,  PbO.  For  all  industrial  glasses  the 
squatting  points  fall  between  750°  and  850°. 

The  range  of  fusibility  for  industrial  glasses,  defined  by 
the  characters  indicated,  is  about  300°  C.,  that  is  to  say,  their 
mechanical  resistance  has  in  that  interval  of  temperature 
exceeded  a  rapid  stress  of  10  g.  to  1000  kg.  per  sq.  cm. 

The  range  of  fusibility  and  its  average  temperature  are 
very  important  for  the  easy  working  of  glass.  The  lower 
the  average  temperature  the  more  slowly  the  glass  cools 
during  the  working,  and  the  greater  the  range  the  greater  the 
number  of  degrees  through  which  the  glass  cools  without 
ceasing  to  be  workable.  The  range  of  fusibility  and  its 
average  temperature  depend  exclusively  on  the  chemical 
composition  of  the  glass.  The  range  increases  with  the 
ratio  of  the  number  of  molecules  of  silica  to  that  of  protoxides 
(R2O  and  RO),  and  the  average  temperature  becomes 
lowered  as  the  basic  oxides  in  the  glass  are  more  fusible 
and  in  greater  proportion.  The  presence  of  lead  in  crystal, 
and  of  high  potash  in  Bohemian  glass,  enabled  3  molecules  of 
silica  to  be  used  for  i  molecule  of  bases,  whereas  in  ordinary 
white  glasses  (rich  in  lime)  the  proportion  cannot  exceed  2-5:1 
without  the  fusibility  being  so  much  reduced  that  the  glass 
could  not  be  worked. 

According  to  S.  English  and  W.  B.  S.  Turner  (/.  Soc. 
Glass  Tech.,  2,  90, 1918),  at  least  three  things  should  be  known 
in  connection  with  the  annealing  of  glass  :  (i)  the  temperature 
at  which  strain  can  be  removed  rapidly  without  causing 
the  glass  to  undergo  deformation  ;  (2)  the  lowest  temperature 
at  which  for  all  practical  purposes  the  strain  can  be  removed  : 
(3)  the  quickest  rate  of  cooling  between  these  two  tempera- 
tures at  which  the  glass  can  safely  be  cooled  without  the 
recurrence  of  strain.  In  the  case  of  three  lead  glasses  con- 
taining 57  to  64  SiO2,  17  to  30  PbO,  and  n  to  15  per  cent.  K2O 
and  Na2O,  the  annealing  temperatures  were  found  to  be 
practically  the  same,  that  with  17  per  cent.  PbO  annealing 
quickly  at  460°,  and  the  other  two  at  450°  C.  Three  lime 
alkali  glasses  with  about  73  per  cent.  SiO2,  7  per  cent.  CaO, 


SILICA   AND  THE  SILICATES 

and  1 8  per  cent.  Na2O  and  K2O,  have  annealing  temperatures 
very  close  to  550°  C.  Two  boric  oxide  glasses  of  practically 
the  same  composition,  65  per  cent.  SiO2,  23  per  cent.  B2O3, 
2  per  cent.  A12O3,  and  10  per  cent.  (Na2O,  K2O),  had  anneal- 
ing temperatures  585°  to  590°  C.  Chemical  glassware 
containing  64  per  cent.  SiO2,  10  A12O3,  7  CaO,  7  B2O3, 
ii '5  (Na2O,  K2O),  annealed  at  630°.  Another  chemical 
glass  containing  70  per  cent.  SiO2,  10  A12O3,  2  ZnO,  7  B2O3, 
8  to  9  (Na2O,  K2O),  annealed  at  635°.  A  third  chemical 
glass,  with  rather  less  alumina,  only  2  B2O3,  but  6  per  cent, 
more  alkalies,  annealed  readily  at  570°  C.,  and  a  soft  soda 
glass  (for  lamp  working)  annealed  quickly  at  530°  C.  The 
same  authors  (/.  Soc.  Glass  Tech.,  3,  125,  1919),  in  reporting 
results  of  an  investigation  on  the  annealing  temperatures 
of  lime-soda  glasses,  point  out  that  whilst  lead  glasses  seem 
to  be  readily  annealed,  chemical  glassware  requires  a  high 
annealing  temperature  followed  by  slow  cooling,  in  order  to 
remove  strain  and  prevent  its  recurrence.  They  found  that 
glasses  with  high  soda  content  can  be  annealed  at  a  com- 
paratively low  temperature  (though  higher  than  for  lead 
glasses),  and  that  they  soften  with  comparative  readiness 
like  lead  glasses.  As  soda  decreases  and  lime  increases  the 
annealing  temperature  rises.  Thus  the  different  soda-lime 
glasses  cannot  be  treated  alike  in  the  lehr,  and  any  change  of 
batch  composition  involves  a  corresponding  change  in  the 
heat  applied  in  order  to  produce  effective  annealing,  as  a 
variation  of  5°  greatly  influences  the  speed  of  annealing. 


CLASSIFICATION. 

Industrially  only  a  small  proportion  of  the  different 
mixtures  possible  are  used.  They  are  chosen  so  as  to  com- 
bine the  greatest  number  of  desirable  qualities,  and  par- 
ticularly a  reasonable  price.  The  qualities  to  be  considered 
include  transparency  and  absence  of  colour,  slight  chemical 
changeability,  mechanical  resistance,  certain  optical  pro- 
perties, etc. 

Nearly  all  industrial  glasses  belong  to  one  of  a  small 


GLASS  AND  ENAMELS  279 

number    of    main    types ;     Noel    Heaton's    classification, 
slightly  modified,  is  as  follows  : — 

A.  Glass  with  only  silica  as  the  acid  constituent,  com- 
bined with  an  alkali  and  one  or  more  other  bases. 

1.  Soda-lime  glass  or  white  glass  (often  called  "  crown 
glass  ")  is  much  used  for  window  glass,  chemical  glassware, 
bottles,  etc.     lyime  is  sometimes  partly  replaced  by  barium 
oxide,    and   a  little   alumina   is   commonly   present.     The 
silica  is  usually  about  69  to  72  per  cent.,  the  composition 
approximating  to  2'5SiO2,  o'5CaO,  o'5Na2O  (which  corre- 
sponds to  71*5  per  cent.  SiO2,  13  per  cent.  CaO,  and  15  per 
cent.    Na2O).     Soda-lime-alumina   glass   is    a   modification 
with  a  substantially  increased  alumina  content,  and  is  used 
chiefly  for  beer,  wine,  and  spirit  bottles,  owing  to  its  strength 
and  insolubility.     The  silica  is  58  to  69  per  cent.,  with 
alumina  about  12  to  3  per  cent. 

2.  Potash-lime    glass,    generally    known    as    Bohemian 
glass,  and   used  for  making  hollow-ware,  or  crown  optical 
glass,  the  latter  having  more  lime  and  less  silica.     The  silica 
is  usually  about  72  per  cent,  or  higher,  the  composition  some- 
times approximating  to  2'5SiO2,  o'5CaO,  O'5K2O. 

3.  Potash-lead    glass,    generally    called    flint    glass    or 
crystal  glass.     This  is  much  used  for  making  hollow-ware, 
bottles,  and  for  optical  glass.     The  composition  sometimes 
approximates    to    3SiO2,    o*5PbO,   O'5K2O   [or    0-5 (K2O, 
Na2O)].     The   theoretical  percentage   composition   of  this 
normal  glass  is  53-2  of  SiO2,  32*9  of  PbO,  and  13-9  of  K2O. 
In  some  varieties  the  lead  oxide  is  substantially  increased  at 
the  expense  of  the  silica  and  potash,  giving  a  denser  glass. 
In  one  variety  of  flint  glass,  barium  oxide  and  zinc  oxide  are 
the  chief  bases,  and  there  is  a  little  boric  oxide  as  well. 

B.  Glass  containing  silica  and  also  other  acid  oxides  : 

4.  Borosilicate  "crown"  glass.     lyike  i,  but  with  B2O3 
partly  replacing  silica.     Used  for  optical  glass,  thermometer 
tubing,  boiler  gauges,  and  laboratory  ware.     May  contain 
72  per  cent,  of  silica,  12  of  B2O3,  5  of  A12O3,  n  of  Na2O. 

5.  Borosilicate  flint  glass.     lyike  3,  but  with  B2O3  partly 
replacing  silica.     Used  for  optical  glass,  etc.,  like  No.  4, 


280  SILICA   AND   THE  SILICATES 

may  contain  67-5  per  cent,  of  SiO2,  2  of  B2O3,  2-5  of  A12O3, 
14  of  Na2O,  7  of  ZnO,  7  of  CaO.  A  glass  transparent  to 
X-rays  consists  of  39-6  per  cent,  of  SiO2,  30  of  B2O3,  20  of 
A12O3,  and  10  of  Na2O. 

6.  Phosphatic  glass,  with  P2O6  partly  replacing  silica. 
C.  Glass  containing  no  silica. 

7.  Borate  and  phosphate  glasses  are  sometimes  used  in 
optical  work.     Phosphate   crown  glass  contains  56-6  per 
cent,  of  P2O5,  5  of  B2O3,  11-3  of  A12O3,  14-8  of  K2O,  and 
1 1 *6   of  MgO.      Borate  flint  glass   contains  66*3  per  cent, 
of  B2O3,  ii  of   A12O3,  4-12  of   Na2O,  6*87   of  PbO,  and 
12 '6  of  ZnO.     A  special  glass  (mentioned  bylye  Chatelier), 
which  contains  no  alkali,  is  very  slightly  attacked  by  acids 
and  only  slightly  sensitive  to  temperature  changes,  contains 
68  per  cent,  of  SiO2,  13  of  B2O3,  37  of  A12O3,  12  of  BaO,  and 
37  of  ZnO.     It  is  useful  for  chemical  purposes. 

8.  Soluble   glass  or  water-glass  is  soluble  in  water.     It 
may  contain  about  60  per  cent,  of  silica  and  40  per  cent,  of 
Na2O,  but  the  composition  varies  to  some  extent,  and  some- 
times K2O  replaces  Na2O,  partly  or  wholly.     Water-glass 
has  already  been  considered  in  Section  II. 

9.  Quartz  glass,  consisting  of  pure  silica  in  the  amorphous 
state.     Quartz  glass  was  considered  in  Section  I. 

According  to  Tscheuschner,  Weber,  Benrath,  and  others, 
the  composition  of  alkali-lime  glass  can  be  expressed  by  the 
formula  *R2O+jyRO+;?SiO2.  For  the  normal  composition 
(the  so-called  Tscheuschner  normal  formula),  x=y=i,  and 
z=6=3(x+y).  Judging  from  Weber's  analyses  of  49  glasses 
(quoted  by  Tscheuschner)  the  relationship  z=$(x-\-y)  is 
only  correct  when  x—y,  that  is,  with  molecular  equivalents 
of  alkalies  and  RO  oxides  equal ;  otherwise  the  value  is  too 
high  with  low  alkali  and  too  low  with  high  alkali,  and  in 
these  cases  better  results  are  obtained  by  substituting 

(x2      \ 
3\—+y)  for  3(x+y),  the  composition  of  the  glass  being 

then   expressed   as   *R2O+;yRoW-  -hv)siO2.    On   this 
basis,  for  y=i  in  Weber's  analyses,  x  can  range  from  0*6  to 


GLASS  AND  ENAMELS  281 

i  for  plate  glass,  from  1-5  to  2  for  Bohemian  glass,  and  from 
0*8  to  1*5  in  glass  for  hollow- ware. 

C.  J.  Peddle  (J.  Soc.  Glass  Technology,  1920),  after  a 
prolonged  investigation,  has  come  to  the  conclusion  that  the 
best  glasses  of  this  type  approximate  to  the  formula  R2O, 
RO,  5SiO2,  or  0'5R2O,  0'5RO,  2'5SiO2.  This  finds  indepen- 
dent confirmation  in  the  compositions  of  St.  Gobain  glasses 
quoted  by  L,e  Chatelier,  viz.  o'37Na2O,  o'63CaO,  2'6SiO2; 
o'47Na2O, 0'53CaO,  2-45  SiO2  ;  o*47Na2O,  o^PbO,  2'45SiO2, 
In  the  case  of  lead  glasses  (crystal)  the  silica  may  be  increased 
to  3  (instead  of  2*5),  which  answers  to  Benrath's  formula  for 
"  normal "  crystal  glass,  viz.  K10Pb7  Si36O84  (or  5K2O, 
7PbO,  36Si02). 

The  most  important  properties  of  glass  are  its  trans- 
parency and  its  rigidity  at  ordinary  temperatures — passing 
into  plasticity  at  high  temperatures  and  fluidity  at  still 
higher  temperatures.  This  combination  of  properties  is 
common  to  only  a  few  colloidal  substances,  and  only  vitreous 
materials  are  also  hard  and  fairly  resistant  to  chemical 
changes.  Water  and  dilute  acids  have  a  slight  action  on 
most  kinds  of  glass ;  even  a  moist  atmosphere  produces 
sooner  or  later  slight  surface  decomposition,  especially  with 
lead  glasses,  or  glasses  containing  too  much  soda,  potash, 
or  boric  acid.  Hydrofluoric  acid  readily  attacks  glass  as 
well  as  other  silicates,  forming  gaseous  silicon  fluoride,  an 
action  which  is  utilized  in  etching.  In  the  hot  plastic  con- 
dition glass  is  ductile  and  malleable,  and  can  be  welded, 
cut,  drawn,  pressed,  etc.  Glass  cracks  under  sudden  changes 
of  temperature,  a  character  which  sometimes  finds  practical 
application  during  manufacture. 

Ordinary  glass  is  to  a  great  extent  opaque  to  both  ultra- 
violet and  infra-red  rays,  but  the  absorption  varies  greatly 
according  to  composition,  being  least  in  pure  quartz  glass 
and  greatest  in  dense  glasses  containing  lead  oxide  (the  latter 
being  opaque  to  X-rays) .  The  density  of  glass  varies  with 
the  molecular  weights  of  the  constituents,  ranging  from  2*25 
for  the  lightest  borate  glasses  to  6*33  for  the  heaviest  lead 
and  barium  glasses.  The  average  density  for  alkali-lime 


282  SILICA   AND   THE  SILICATES 

glass  is  2*5,  and  that  for  ordinary  flint  glass  is  3-0.  Hardness 
and  resistance  to  abrasion  is  proportional  to  the  content  of 
silica,  lime,  and  alumina,  and  inversely  proportional  to  the 
content  of  lead  and  alkali.  Soda  glasses  are  generally  harder 
than  the  corresponding  potash  glasses. 

Glass  is  a  poor  conductor  of  heat,  and  a  non-conductor 
of  electricity  (except  when  moist).  It  breaks  with  a 
conchoidal  fracture. 


RAW  MATERIALS. 

Silica  is  chiefly  used  in  the  form  of  sand.  lyong  ago 
(in  the  seventeenth  century)  calcined  and  ground  flints 
were  used  for  making  the  best  glass,  and  this  gave  rise  to 
the  name  "  flint  glass."  Though  the  name  persists,  the 
use  of  flints  for  this  purpose  was  abandoned  when  com- 
paratively cheap  supplies  of  pure  sands  became  available ; 
those  of  Fontainebleau  and  L/ippe  (for  instance)  are  almost 
entirely  composed  of  silica.  Sometimes  crushed  sandstone 
is  used,  and  in  exceptional  cases  ground  quartz.  Fine  white 
sand  from  certain  British  localities,  as  L,ynn,  Alum  Bay 
(Isle  of  Wight),  and  Aylesbury,  have  been  much  used  in 
recent  years.  For  the  best  glass,  the  sands  used  should 
contain,  besides  silica  with  a  little  alumina  and  iron  oxide, 
preferably  only  traces  of  other  substances.  The  iron  oxide 
should  always  be  under  0*1  per  cent.,  and  is  often  under 
0*04  per  cent.  Alumina  may  for  some  glasses  amount  to 
3  or  4,  or  even  as  much  as  8  per  cent.  In  the  absence  of 
alumina,  the  silica  should  be  at  least  98  per  cent.,  and  for 
the  best  glasses  should  be  99*5  per  cent,  or  more.  In  this 
connection,  however,  a  statement  of  R.  L.  Frink  (Trans. 
American  Ceramic  Soc.,  //,  296,  1909)  may  be  mentioned, 
that  a  certain  American  factory  produced  glass  of  the  highest 
quality  from  a  low-grade  sand  containing  88*51  per  cent, 
silica,  with  7*26  alumina  and  1*07  calcium  carbonate,  etc., 
and  when  under  new  management  a  pure  sand  with  98*89  per 
cent,  silica  was  substituted,  though  the  quality  and  quantity 
of  the  product  remained  substantially  the  same,  the  low- 


GLASS  AND  ENAMELS  283 

grade  sand  produced  a  glass  far  superior  in  physical  pro- 
perties to  that  made  with  the  purest  sand.  As  regards 
mechanical  composition  of  sands  (or  crushed  rock)  for  glass- 
making,  it  is  desirable  to  have  as  large  a  proportion  as  possible 
of  nearly  the  same  size,  preferably  the  medium  grade 
(diameter  between  0-5  and  0*25  mm.)  or  the  next  smaller 
grade  (diameter  between  0-25  and  o'l  mm.).  Smaller 
particles  (less  than  O'l  mm.  diameter)  of  silt  and  clayey 
material  are  injurious,  owing  to  their  "  blowing  out  "  in 
the  batch,  to  producing  "  cordiness  "  by  differential  melting, 
to  entangling  air  bubbles,  and  to  being  more  difficult  to  melt. 
Particles  larger  than  0-5  mm.  diameter  should  be  sifted  out 
if  present.  As  to  mineral  composition,  good  glass-sands 
consist  almost  entirely  of  quartz  grains ;  alumina  may  be 
present  in  the  form  of  felspar  or  kaolin  ;  heavy  minerals 
like  magnetite,  hematite,  limonite,  ilmenite,  sphene,  rutile, 
zircon,  should  only  occur  in  small  quantities. 

Prof.  Boswell  lays  down  the  following  for  guidance  in 
the  use  of  sands  for  glass-making  :  For  the  commonest  glass 
(bottle-ware,  etc.)  the  sand  must  be  well  graded,  and  com- 
posed of  grains  of  suitable  size.  The  iron  oxide  may  range 
up  to  i  per  cent.,  the  presence  of  small  amounts  of  titanium 
oxide,  alumina,  lime,  and  alkalies  are  of  little  importance, 
but  the  silica  should  be  fairly  high. 

For  medium-class  glassware  (flint-glass  bottles,  chemical 
ware,  globes,  chimneys,  pressed  table-ware,  etc.)  high  silica 
and  low  iron  content  are  required,  with  high  alumina  for 
much  laboratory  ware,  etc.  The  size  of  grain  should  not  be 
more  than  0-5  mm.  nor  less  than  O'l  mm.  diameter. 

For  high-class  ware  (optical  glass,  table  glass  for 
"  cutting,"  and  other  special  glasses)  high  silica  with  little 
or  no  iron  oxide  are  required,  and  sometimes  high  alumina. 
As  the  batch  is  sometimes  powdered,  and  the  mixture  kept 
melted  for  a  long  time  (usually  with  stirring),  crushed  rocks 
and  quartz,  and  poorly  graded  sands,  can  be  used. 

Fine  silicious  material  undoubtedly  introduces  numerous 
minute  air  bubbles,  which  are  not  entirely  removed  by 
stirring  or  during  the  final  "  fining."  It  is  questionable 


284  SILICA   AND   THE  SILICATES 

whether  stirring  ever  produces  the  homogeneity  obtained 
by  using  well-graded  and  evenly-melting  sands. 

Boswell  gives  the  limits  of  iron  content  for  some  glasses 
as  follows  :  optical  glasses  containing  barium  and  zinc,  below 
0-02  per  cent. ;  "  crown  "  optical  glass,  up  to  0-04  per  cent.  ; 
table  glass  ("  cut  "  and  "  crystal "  ware),  0*02  per  cent. ; 
laboratory  and  medical  ware  0*04  or  0*05  per  cent. ;  plate 
glass,  0*05  per  cent. ;  window  glass,  O'i  per  cent.  The 
possible  presence  of  iron  in  the  limestone,  felspar,  manganese, 
etc.,  should  not  be  overlooked. 

Many  British  sands  contain  2  to  3  per  cent,  alumina. 
In  a  few  cases  as  much  as  10  to  18  per  cent,  alumina  is 
present,  as  in  the  refractory  sands  of  Derbyshire  and 
Staffordshire.  A  small  proportion  of  alumina  tends  to 
strengthen  glass,  making  it  tougher  and  more  resistant  to 
pressure.  Sands  with  a  high  percentage  of  alumina  are 
valuable  for  glasses  used  for  making  thermometers,  gauge 
glasses,  combustion  tubing,  and  other  resistant  glasses. 

As  regards  British  glass-making  sands,  the  sands  of  the 
shore  and  dunes  are  only  useful  for  bottle  glass,  and  Thanet 
sands  are  also  used  only  for  bottle  glass ;  even  sands  made 
by  pulverizing  blast-furnace  slag  have  been  used  for  this 
purpose,  and  natural  igneous  rocks  such  as  basalt  have  been 
similarly  used.  Aylesbury  sand  can  be  used  for  optical 
glass  and  many  other  varieties.  I,eighton  and  I,ynn  sands 
are  used  for  all  kinds  of  lighting  glass  (electric  globes, 
chimneys,  etc.),  laboratory  ware,  pressed  ware,  flint-glass 
bottles,  etc.  Aylesford  and  Reigate  sands  are  good  enough 
for  something  better  than  bottle  glass.  Some  of  the  York- 
shire sands  (Huttons  Ambo,  etc.)  when  washed  are  suitable 
for  nearly  all  the  better  kinds  of  glass.  The  best  Irish 
silicious  material  is  the  crushed  quartzite  of  Muckish 
Mountain  (Donegal). 

Glass  containing  about  0*02  per  cent,  iron  oxide  is  almost 
colourless,  a  glass  with  0*1  per  cent,  iron  oxide  is  quite  green, 
and  with  i  per  cent,  it  is  dark  green. 

For  the  best  glasses,  British  manufacturers  used  Fontaine- 
bleau  sand  (from  near  Paris),  which  is  high-class,  with 


GLASS  AND  ENAMELS  285 

generally  less  than  0-03  per  cent,  iron  oxide,  with  even  grain 
(favouring  production  of  homogeneous  glass),  and  angular 
grains  (ensuring  quick  melting),  uniform  in  quality  and 
obtainable  regularly. 

C.  J.  Peddle  (J.  Soc.  Glass  Technology,  i,  27, 1917)  reports 
that  with  thorough  washing  several  British  sands,  etc.,  can 
be  used  advantageously  for  glass-making ;  the  crushed 
quartzite  of  Muckish  Mountain  will  give  a  good  glass  even 
without  washing.  Aylesbury  and  I,ynn  sands,  with  those 
from  HuttonsAmbo  and  Burythorpe  (both  in  Yorkshire), 
will  all  give  colourless  glasses  when  washed,  and  with 
manganese  dioxide  as  decolorizer,  should  certainly  be  good 
enough  for  flint  glass.  He  is  satisfied  that  with  reasonable 
treatment  many  British  sands  could  successfully  replace 
Fontainebleau  sand  for  many  kinds  of  good  glass. 

Alkali  was  in  olden  times  used  in  the  form  of  native 
sodium  carbonate.  Afterwards  the  ashes  of  plants  were 
used,  and  later  manufactured  sodium  carbonate  (from  salt). 
Sodium  sulphate  was  used  instead  of  the  carbonate— first 
in  France  in  1825,  and  in  England  in  1831 — and  is  now  in 
extensive  use.  It  is  customary,  but  not  necessary  if  the 
temperature  be  high  enough,  to  add  carbon  (as  anthracite 
coal  or  charcoal)  to  deprive  the  sulphate  of  oxygen  and  so 
facilitate  its  decomposition,  but  this  can  only  be  done  with 
leadless  glasses.  It  is  advisable  to  reduce  the  sulphate  to 
sulphite,  not  to  sulphide  (which  would  give  the  glass  a  yellow 
colour). 

Though  sodium  sulphate  is  much  cheaper  than  the 
carbonate,  it  should  be  remembered  that  53  parts  by  weight 
of  the  latter  are  equal  to  71  parts  of  the  former,  and  the 
sulphate  needs  a  much  higher  temperature  than  the  carbonate 
to  react  with  silica. 

Potash  is  invariably  used  as  carbonate,  generally  known 
as  pearl-ash.  In  lead  glass  and  certain  coloured  glasses, 
part  of  the  potash  is  used  as  nitrate,  mainly  to  obviate  any 
tendency  to  reduction. 

Lime  is  generally  introduced  as  carbonate,  or  hydrated 
oxide,  both  being  obtained  from  chalk,  or  limestone.  Chalk  is 


286  SILICA   AND  THE  SILICATES 

contaminated  with  flints,  which  are  only  objectionable  when 
large,  some  of  the  silica  then  possibly  remaining  undissolved, 
causing  opaque  enclosures  (called  "  stones  ")  in  the  finished 
glass.  In  limestone,  magnesia  is  generally  objectionable, 
as  it  tends  to  make  the  glass  hard  and  viscous. 

Barium  Oxide  is  used  as  precipitated  carbonate. 

Alumina  is  mostly  present  as  an  impurity,  being  dis- 
solved by  the  molten  glass  from  the  fireclay  vessels.  In  the 
hard  glasses,  alumina  is  added  in  the  hydrated  condition. 
In  opal  glasses  it  is  added  in  the  form  of  cryolite,  but  this 
is  used  mainly  on  account  of  the  fluorine. 

Manganese  is  generally  added  as  the  dioxide.  It  is 
essentially  a  colouring  oxide,  but  is  much  used  to  counteract 
the  coloration  caused  by  iron,  thus  making  the  glass  white, 
or  rather  colourless. 

Gullet  (which  is  waste  or  broken  glass)  is  invariably  added 
as  well  as  the  other  ingredients,  partly  to  use  up  the  bits 
left  over,  and  partly  to  facilitate  chemical  reaction  at  lower 
temperatures,  that  is,  it  acts  as  a  flux.  The  cullet  melts,  and 
dissolves  both  alkali  and  silica.  The  cullet  used  should 
closely  correspond  in  composition  to  the  kind  of  glass  it  is 
intended  to  make  with  it,  and  should  be  as  far  as  practicable 
free  from  iron  in  any  condition. 


MIXING. 

The  mixing  of  the  materials  is  very  important,  for  unless 
it  is  done  well,  irregularities  may  easily  arise  in  the  resulting 
glass.  Sometimes  the  mixing  is  done  by  hand,  the  powdered 
raw  materials  being  turned  over  by  a  workman  with  a  wooden 
shovel,  and  afterwards  passed  through  a  sieve  until  well 
mixed.  More  usually  a  mechanical  mixer  is  employed. 
It  is  a  curious  result  of  experience  that,  when  the  batch  is 
mixed  mechanically,  China  clay  may  alwa)^s  be  used  as  a 
source  of  alumina,  but  with  hand-mixing  a  batch  containing 
China  clay  will  never  produce  a  satisfactory  glass.  Dr. 
Travers  believes  materials  like  China  clay  can  be  advan- 
tageously ground  with  one  of  the  other  constituents  of  the 


GLASS   AND  ENAMELS  287 

batch.  He  also  concludes  that  it  is  distinctly  advantageous 
to  grind  the  cullet  and  mix  it  with  the  batch,  instead  of 
following  the  usual  practice  of  adding  it  separately  at  the 
outset. 

Wet  mixing  of  the  batch  has  been  recommended,  on  the 
ground  of  securing  more  intimate  mixture,  the  material  to 
be  afterwards  dried  by  some  of  the  waste  heat  from  the 
furnaces. 

The  proportions  of  the  constituents  must  largely  depend 
on  the  temperature  of  firing,  the  time  of  exposure  of  pots 
at  different  temperatures,  and  the  composition  and  amount 
of  cullet  added  to  the  batch.  When  sodium  sulphate  is 
used,  a  substantial  amount  is  removed  by  skimming,  for  it 
does  not  all  react  with  silica.  Some  alkali  passes  off  as 
vapour,  and  so  is  lost  to  the  glass. 


DECOLORIZING. 

This  is  the  process  of  correcting  the  undesirable  tints 
given  by  small  amounts  of  iron  occurring  as  impurity,  by 
adding  small  amounts  of  other  colouiing  substances  such  as 
manganese  dioxide,  nickel  oxide,  selenium,  etc.,  to  neu- 
tralize the  colouring  effect  of  the  iron.  Iron  may  be  derived, 
not  only  from  impurities  in  the  raw  materials,  but  also  by 
solution  of  the  fireclay  from  the  vessels  containing  the  molten 
glass  (or  "metal,"  as  it  is  called),  and  tends  to  produce  a 
greenish  tint.  It  should  be  remembered  that  sodium  silicate 
in  glass  tends  to  communicate  a  bluish-green  tint,  and  lead 
silicate  is  distinctly  yellowish.  Dr.  B.  W.  Washburn  showed 
that  even  iron  in  the  furnace  atmosphere  can  colour  glass. 
Manganese  dioxide  colours  glass  purple,  and  this  being 
complementary  to  the  green  colour  from  iron,  a  suitably 
proportioned  application  of  the  former  practically  eliminates 
the  colour  due  to  iron.  This  effect  of  manganese  dioxide 
is  only  produced  under  oxidizing  conditions,  and  if  a 
sufficiency  of  oxidizing  agent  (or  agents)  be  not  present  in 
the  glass  batch  the  purple  colour  is  unstable  and  its  de- 
colorizing effect  is  not  to  be  relied  on.  In  mixtures  for 


288  SILICA   AND  THE  SILICATES 

crystal  glass,  strong  oxidizing  agents  are  therefore  to  be  used, 
generally  in  the  form  of  nitre  (saltpetre)  and  red  lead,  both 
of  which  give  off  oxygen  when  heated.  If  the  melted  glass 
be  kept  very  hot  for  a  long  time  there  is  a  possibility  of  all 
the  liberated  oxygen  being  driven  off,  after  which  further 
heating  would  tend  to  make  the  glass  greenish  again.  If 
by  addition  of  too  much  manganese  dioxide  the  glass  becomes 
distinctly  purple,  a  little  carbonaceous  material  (in  the  form 
of  a  bit  of  charred  wood  or  potato)  can  be  pushed  to  the 
bottom  of  the  glass  by  a  forked  rod,  and  the  resulting 
vapours  soon  discharge  the  purple  colour.  Pure  manganese 
dioxide  should  be  used,  as  the  native  pyrolusite  (often  used 
as  a  decolorizer)  is  apt  to  contain  substantial  proportions 
of  iron  oxide,  which  is  of  course  highly  objectionable. 

Nickel  oxide  has  also  been  used  as  a  decolorizer,  and 
selenium  or  alkaline  selenites  01  selenates.  In  some  cases 
preparations  containing  cobalt  have  been  used  for  the  pur- 
pose of  neutralizing  colour  in  glass  (particularly  when  it 
tends  to  be  yellowish),  but  only  a  very  small  amount  is 
needed. 

According  to  W.  Frommel  (Keramische  Rundschau,  1917), 
the  green  colouring  effect  of  i  kg.  iron  is  compensated  by 
0*208  g.  selenium  (or  0*0208  per  cent.),  and  as  a  glass  batch 
rarely  contains  more  than  0*2  per  cent,  iron  oxide,  the 
selenium  necessary  would  not  usually  exceed  0*00416  per 
cent,  of  the  batch,  or  rather  more  than  double  that  amount 
of  sodium  selenite. 

The  yellow  colour  imparted  to  glass  by  ferric  oxide  is 
much  less  intense  than  the  greenish  tint  imparted  by  ferrous 
oxide,  and  advantage  is  sometimes  taken  of  this  fact  by 
adding  instead  of  a  colouring  substance  (such  as  manganese 
dioxide)  an  oxidizing  substance  to  ensure  that  the  iron  shall 
be  in  the  ferric  condition.  Oxide  of  arsenic  is  used  in  this 
way,  and  the  liberated  arsenic,  being  volatile,  seems  to 
escape  into  the  flues,  though  in  some  cases  indications  of  the 
higher  oxide  of  arsenic  have  been  found  in  the  resulting  glass. 
Nitre  and  similar  oxidizing  agents  have  a  similar  effect  on 
iron  in  glass,  though  the  main  object  of  adding  a  little  nitre 


GLASS  AND  ENAMELS  289 

is  generally  to  destroy  carbonaceous  matter,  and  to  assist 
in  preventing  premature  volatilization  of  arsenic. 

In  connection  with  the  use  of  a  colouring  substance  as  a 
decolorizer,  it  should  not  be  forgotten  that  the  result  is  only 
attained  through  the  loss  of  a  portion  of  the  light  which 
would  be  transmitted  by  really  colourless  glass.  For  this 
reason  that  device  cannot  be  adopted  for  optical  glass,  in 
the  manufacture  of  which  the  purest  materials  available 
should  be  used  so  as  to  avoid  any  such  loss  of  light.  If  it 
should  still  show  some  colour,  it  is  not  advisable  to  attempt 
to  neutralize  it  by  the  use  of  a  decolorizer. 


FURNACES. 

Both  furnaces  and  pots  are  made  of  refractory  fireclays, 
which  occur  chiefly  in  the  districts  about  Glasgow,  I/eeds, 
Stourbridge,  and  other  Midland  and  North  of  England 
centres,  as  well  as  in  foreign  countries  (Belgium,  France, 
Germany,  etc.).  The  dry  weathered  clay  is  ground,  sifted, 
mixed  with  grog  (consisting  of  ground  burned  fireclay  or 
old  pots),  tempered  with  water,  and  the  plastic  mixture  used 
for  making  into  blocks  or  pots,  etc.  The  grain  size,  especially 
of  the  grog,  should  be  adapted  for  the  purpose  to  be  served. 
For  resisting  sudden  temperature  changes,  coarse  grains 
answer  best,  but  to  resist  the  chemical  action  of  molten  glass 
or  slag,  fine  grained  material  is  better.  Often  it  will  be 
advantageous  to  combine  the  two  in  various  degrees. 
Furnace  blocks,  when  shaped,  are  slowly  dried,  and  finally 
fired  in  much  the  same  way  as  other  fireclay  goods,  preferably 
up  to  cone  14,  but  never  below  cone  12. 

The  essential  constituents  of  the  fireclays  are  silica  and 
alumina,  and  the  latter  should  not  be  less  than  30  per  cent., 
whilst  the  silica  may  range  from  about  45  to  65  per  cent. 
The  other  constituents  are  more  or  less  accidental  impurities, 
which  mostly  act  as  fluxing  agents.  The  percentage  of  ferric 
oxide  should  not  exceed  1*5,  lime  0*5,  magnesia  0*2,  and  total 
alkalies  (potash  and  soda)  about  2. 

The  plasticity  of  the  fireclay  should  be  increased  as  much 
c.  19 


290  SILICA   AND   THE  SILICATES 

as  practicable  by  weathering,  after  removal  of  all  injurious 
foreign  materials,  such  as  pyrite,  stones,  vegetable  matter, 
etc.  It  is  afterwards  treated  as  described  above. 

The  glass  furnace  is  a  heated  chamber,  in  which  the  glass- 
house pots  are  set.  In  the  old  round  furnace,  commonly 
used  in  England  until  comparatively  recent  times,  and  still 
largely  used,  six  or  twelve  pots  are  arranged  in  a  circle 
on  the  "  siege,"  a  kind  of  hob  forming  the  floor  of  the  furnace. 
The  fire  is  placed  below,  and  the  flames  rise  through  the 
"  eye  "  of  the  furnace — centrally  situated  as  regards  the 
chamber,  and  below  the  level  of  the  siege — to  heat  the  pots. 
The  fire  burns  in  a  round  box  with  a  bottom  formed  of  strong 
iron  bars.  Passing  right  across  below  the  furnace  is  the 
"  cave/'  an  underground  tunnel,  through  each  end  of  which 
air  is  drawn  by  the  action  of  the  draught  in  the  chimney 
cone  above  the  furnace.  Coal  is  supplied  to  the  fire  at 
intervals  through  a  passage  under  the  siege  from  the  floor 
of  the  glasshouse  sloping  towards  the  firebox.  Sometimes 
a  mechanical  feeder  is  used,  fitted  below  the  firebars.  The 
crown  of  the  furnace,  supported  by  pillars  of  firebricks, 
covers  the  pots  and  the  siege.  From  this  crown  the  flames 
are  reflected  down  upon  the  pots,  and  passing  between  the 
pots  they  escape  by  small  openings  connecting  the  siege  with 
small  flues  or  chimneys  leading  upwards  to  the  outer 
chimney  cone. 

Modern  glass-melting  furnaces  are  generally  gas-fired, 
usually  with  regeneration  or  recuperation  of  waste  heat. 
They  are  smaller  and  more  convenient  than  the  old  coal-fired 
furnace,  and  combine  more  complete  combustion  of  the  fuel 
with  easier  regulation  and  increased  production,  being  thus 
more  economical.  They  also  give  much  higher  temperatures. 

The  regenerative  system  was  invented  by  Siemens,  the 
air  or  gas  being  pre-heated  in  regenerators,  that  is,  chambers 
occupied  by  a  checker- work  of  firebricks.  The  gases  from 
the  furnaces  pass  through  two  regenerators  and  heat  the 
bricks,  while  the  gas  and  air  going  to  the  furnace  pass 
separately  through  two  other  regenerators.  At  definite 
intervals  the  direction  of  the  gas  and  air  is  reversed  by  the 


GLASS  AND  ENAMELS 


291 


manipulation  of  two  valves,  so  that  the  gas  and  air  for  the 
furnace  becomes  pre-heated  in  the  regenerators  through 
which  the  hot  waste  gases  have  passed.  Owing  to  the 
periodical  reversal,  the  system  of  heating  is  intermittent. 

In  the  recuperative  system  the  waste  gases  and  the  air 
pass  continuously  in  one  direction,  one  tube  or  passage 
usually  surrounding  the  other,  so  as  to  allow  the  air  to  be 
very  satisfactorily 
pre-heated.  The  gas 
producers  are  usually 
close  to  the  furnaces, 
so  that  the  gas  is  hot 
enough  when  it  enters 
the  latter. 

A  semi  -  gas  -  fired 
system,  in  which  for 
the  burning  of  the 
gases  (produced  by 
the  burning  coal), 
secondary  air,  pre- 
heated to  some  extent 
by  traversing  p  assages 
in  the  furnace  walls, 
is  delivered  at  suit- 
able points  of  the 
furnace,  is  still  in 
vogue  in  many  glass- 
works, both  abroad 


FIG.  22. — Boetius  furnace  (cross  section). 
(By  courtesy  of  Mr.  T.  Teisen.) 

C,  Air  channels. 
E,  Eyes  of  furnace. 
H,  Working  hollows. 
N,  Chimney. 


and  in  England.  The 
Boetius  furnace  is  of 
this  type,  which  is  an 
improvement  on  the  old  coal-fired  furnace,  but  has  the  dis- 
advantage that  the  contents  of  a  broken  pot  will  run  into 
the  fire. 

Several  forms  of  regenerative  glass-melting  furnaces— 
modifications  of  the  Siemens  furnace — are  in  use,  especially 
on  the  Continent,  but,  while  economical  as  regards  fuel, 
they  suffer  from  the  disadvantage  that  owing  to  the 


SILICA   AND   THE  SILICATES 

periodical  reversals  the  temperature  varies,  first  one  end  of 
the  furnace  and  then  the  other  end  being  the  hotter. 

Among  the  better  known  continuous  furnaces  are  the 
Hermansen,  the  Nehse-Dralle,  and  the  Stein,  the  Nehse- 
Dralle  being  mainly  used  in  Germany.  The  Hermansen 
Furnace,  of  Danish  origin,  is  used  in  Scandinavia,  Germany, 
Russia,  England,  and  other  countries,  and  has  proved  very 
satisfactory.  In  the  ordinary  type,  the  burner  is  placed^in  the 


FIG.  23. — Boetius  furnace  (longitudinal  section). 
(By  courtesy  of  Mr.  T.  Teisen.) 

centre  of  the  circular  upper  part  of  the  furnace,  and  the  pots 
are  arranged  in  a  ring  within  the  circular  wall,  between  the 
pillars  which  support  the  crown.  The  flues  are  in  these 
pillars,  near  the  fronts  of  the  pots,  the  latter  being  oval  in 
horizontal  section.  The  recuperator — made  of  tubes  of 
special  shape  and  of  special  fireclay — is  placed  on  each  side 
of  the  producer  in  the  lower  part  of  the  furnace,  fully  pro- 
tected by  cooling  channels  for  the  flow  of  glass.  The  burned 


GLASS  AND  ENAMELS 


293 


gases  pass  through  the  larger  channels  of  the  recuperator,  and 
the  secondary  air  is  pre-heated  on  passing  through  small 
cross  channels  formed  by  projections  from  adjacent  tubes, 
leakage  between  the  products  of  combustion  and  the  air  is 
successfully  avoided.  I^ike  other  modern  recuperative 
furnaces,  the  Hermansen  melts  the  glass  during  the  night, 
ready  for  working  the  next  day,  whereas  the  old  direct  coal- 


FIG.  24. — Hermansen  recuperative  furnace  (for  8  covered  pots). 
F,  Fire  door.  B,  Burner  (eye,  with  2  air  inlets). 

P,  Producer.  R,  "  Hermansen  "  recuperator. 

GP,  Glass  pocket. 

fired  furnaces  are  usually  worked  only  two  or  three  times  a 
week.  The  Hermansen  type  of  furnace  is  much  easier  for 
the  workman  to  tend  than  the  more  complicated  re- 
generative furnaces.  With  greater  units  it  may  be  fired  from 
separate  producers. 


294 


SILICA   AND   THE  SILICATES 


The  Stein  Recuperative  Furnace  has  recuperative 
chambers  on  each  side  of  the  furnace,  so  that  the  flow  of 
glass  from  a  cracked  or  broken  pot  does  not  reach  them. 


The  air  passages  in  the  recuperators  are  vertical,  and  air 
is  constantly  rising  in  them,  and  expanding  as  it  is  heated, 
the  space  round  the  pots  being  quite  filled  with  flame,  thus 


GLASS  AND  ENAMELS  295 

ensuring  uniform  temperature.  The  producers  can  be 
independent,  or  built  with  the  furnaces.  Here  also  much 
greater  economy  of  fuel  is  claimed  than  in  modern  regenera- 
tive furnaces. 

Brief  reference  may  also  be  made  to  a  remarkably  efficient 
type  of  gas-fired  furnace — the  Dennis  Simplex  Furnace — 
described  by  Dr.  M.  W.  Travers  in  /.  Soc.  Glass  Technology, 
4,  205,  1920.  It  was  evolved  in  a  somewhat  haphazard 
fashion  from  a  gas-fired  furnace  of  a  continental  pattern, 
having  two  producers  built  into  the  centre  of  it,  and  flanked 
by  recuperators.  This  furnace  did  not  work  satisfactorily, 
and  when  eventually  it  was  put  out  of  action,  it  was  decided 
not  to  rebuild  it  but  to  reconstruct  it,  the  producers  being 
completely  removed  and  replaced  by  a  Frisbie  under-feed 
grate.  The  new  furnace  thus  obtained  was  found  to  work 
very  efficiently,  and  to  be  exceptionally  easy  to  manipulate. 

The  lehr  (leer,  or  annealing  furnace)  is  in  most  English 
glasshouses  an  arched  chamber  more  than  40  ft.  long,  with  a 
later  ally -placed  firebox  at  the  hot  end.  The  glass  articles 
are  placed  in  pans,  which  are  heated  directly  by  the  fire. 
As  the  pans  are  drawn  down  the  lehr,  the  contents  gradu- 
ally cool  off.  This  method  answers  well  for  annealing  lead 
glassware,  but  is  unsatisfactory  for  resistance  beakers  and 
flasks,  notwithstanding  their  lightness.  Dr.  M.  W.  Travers 
(/.  Soc.  Chemical  Industry,  37,  238T,  1918)  attributes 
this  to  the  presence  of  a  kind  of  skin,  which  may  account 
for  the  fact  that  unless  the  glass  is  thoroughly  soaked 
at  the  annealing  temperature  it  is  not  properly  annealed. 
Scientific  glassware  has  enormous  bulk  for  very  small 
weight,  and  as  the  goods  must  pass  very  slowly  through 
the  hot  part  of  the  lehr,  it  follows  that  this  class  of 
ware  requires  very  large  lehr  accommodation.  The  use  of 
producer  gas  has  been  found  very  effective  with  lehrs  for 
resistance  glass,  as  in  the  Teisen  patent  lehr  illustrated, 
the  gas  entering  a  space  below  the  bed  of  the  lehr, 
which  is  formed  of  a  brick  arch,  and  then  passing  up 
ports  at  the  side.  Secondary  air  enters  through  slides  at 
the  front  and  meets  the  gas  at  the  bottom  of  the  ports  j 


296 


SILICA   AND   THE  SILICATES 


GLASS  AND  ENAMELS  297 

the  length  of  the  flame  issuing  from  the  ports — and  therefore 
the  temperature  of  the  lehr — can  be  increased  or  decreased 
by  closing  or  opening  the  air  slides.  The  temperature  of  the 
hot  part  of  the  lehr  can  be  thus  regulated  to  within  10°,  and 
as  the  hot  gas  from  the  producer  heats  the  bed  of  the  lehr 
from  below,  the  temperature  can  be  maintained  fairly 
uniform.  The  fall  of  temperature  along  the  lehr  is  regulated 
by  taking  away  the  hot  gases  into  a  flue,  through  openings 
controlled  by  dampers.  Resistance  glass  may  be  removed 
from  the  lehr  at  300°  C.  with  safety.  Optical  glass  needs 
very  careful  annealing. 

Very  thin  glass  does  not  require  annealing,  but  thick 
glass  may  need  a  very  long  time.  For  most  ordinary  glass- 
ware about  3  to  6  or  8  hours  are  generally  occupied  in 
passing  it  through  the  lehr.  Very  bulky  articles  are  usually 
put  in  annealing  kilns,  and  often  take  about  3  days. 
The  annealing  of  optical  glass  (especially  the  heavy  lead 
and  barium  glasses)  may  take  as  much  as  10  days.  It 
may  be  noted  that  the  peculiar  behaviour  of  the  well-known 
Rupert's  drops  (or  Dutch  tears)  is  due  to  internal  strains. 

GLASSHOUSE  POTS. 

The  pots  used  for  melting  glass  are  made  from  mixtures 
of  clay  and  grog  prepared  in  the  same  way  as  for  furnace 
blocks.  For  crystal  or  flint  glass,  which  contains  lead  oxide, 
the  melting  batch  has  to  be  protected  from  reducing  agencies, 
especially  the  flames,  smoke,  and  ash  of  the  old  coal-fired 
furnace  chambers,  as  otherwise  metallic  lead  would  be 
formed  and  the  glass  spoiled  ;  hence,  covered  pots  are  used 
for  melting  these  lead  glasses,  each  pot  having  an  opening 
in  front,  through  which  the  materials  are  charged  and  the 
molten  glass  withdrawn.  Covered  pots  are  also  used  for 
optical  glass.  For  most  other  kinds  of  glass,  the  pots  used 
are  open  at  the  top.  In  the  latter  case  the  melting  may  be 
aided  by  directing  the  heat  on  to  the  surface  of  the  materials 
within  the  pot.  In  the  case  of  covered  pots,  the  heat  has  to 
pass  through  the  cover. 

The  pots  are  subject  to  the  corrosive  action  of  their 


298  SILICA   AND   THE  SILICATES 

contents,  and  have  also  to  withstand  the  furnace  heat  and 
the  pressure  of  the  contained  glass.  The  cracking  of  a  pot 
has  serious  results,  as  the  molten  glass  is  not  only  lost,  but 
it  attacks  the  parts  of  the  furnace  that  it  comes  into  contact 
with,  and  on  reaching  the  firebox  it  combines  with  the  fuel 
ash  and  soon  interferes  with  the  draught. 

The  proportion  of  grog  to  clay  in  the  mixture  depends 
on  the  plasticity  of  the  clay,  but  usually  as  much  grog 
is  incorporated  as  is  practicable,  and  it  may  vary  from 
about  6  volumes  of  grog  to  5  of  clay,  to  i  volume  of  grog 
to  3  of  clay.  After  mixing  with  water,  and  soaking  for 
some  time,  the  mixture  is  tempered  by  treading  well  with 
bare  feet,  which  seems  to  consolidate  the  clay  mass  better 
than  any  mechanical  treatment.  It  is  then  left  to  mature 
for  a  few  weeks  before  being  used,  and  it  is  ready  after  this 
for  making  into  pots.  Various  shapes  and  sizes  are  used, 
but  the  commonest  is  a  round  pot  38  ins.  in  diameter  and 
42  ins.  high.  In  making  a  pot,  the  workman  arranges  little 
rolls  of  clay  on  a  round  flat  board  of  the  same  size  as  the 
bottom  of  a  pot.  These  rolls  are  laid  in  circles,  and  pressed 
together  so  as  to  exclude  air,  until  a  circular  slab  about  4  ins. 
thick  is  formed,  which  is  made  smooth.  The  sides  or  walls 
of  the  pot  are  built  upon  the  slab  in  the  same  way  from  little 
rolls,  to  a  thickness  of  about  3  ins.  The  wall  is  built  up  about 
6  ins.  at  a  time,  with  intervals  of  a  day  or  so  to  enable  each 
section  to  stiffen  a  bit  before  the  next  section  is  added. 
When  a  height  of  about  30  ins.  has  been  reached,  a  clay  ring 
about  1 8  ins.  in  diameter  is  put  in  the  pot.  This  clay  ring 
afterwards  floats  on  the  melted  glass,  and  keeps  scum  away 
from  the  middle  portions.  After  this,  the  potmaker  begins 
to  draw  the  remaining  portions  of  the  sides  inwards  to  form 
the  cover  of  the  pot,  at  the  same  time  reducing  the  thickness 
of  the  walls.  The  working  opening  is  made  by  cutting  out 
the  clay  while  it  is  soft.  These  openings  are  closed  by 
stoppers  while  the  glass  is  being  melted.  The  pots  are 
dried  very  gradually  over  a  period  of  several  months.  Open 
pots  are  made  in  the  same  way,  but  are  finished  off  at  the 
proper  height  before  putting  to  dry. 


GLASS  AND  ENAMELS  299 

Mechanical  contrivances  of  different  kinds — a  hydraulic 
press,  etc. — have  been  tried  for  shaping  pots  with  the  aid  of 
moulds,  but  with  no  great  success.  Better  results  have  been 
obtained  by  casting,  the  whole  pot  or  parts  being  made  by 
pouring  prepared  slip  into  the  mould.  For  this  purpose 
the  slip  is  made  very  heavy,  though  quite  mobile,  by  adding 
to  the  prepared  clay  a  little  sodium  carbonate  and  sodium 
silicate  along  with  a  limited  amount  of  water.  In  the 
United  States  this  has  been  combined  with  the  use  of  a 
special  grog,  having  a  composition  resembling  that  of  hard 
porcelain,  for  pots  intended  for  optical  glass ;  this  proved 
more  resistant  than  ordinary  grog  to  the  molten  glass.  In 
this  country,  a  modified  casting  process,  patented  by  Allen 
and  Ames,  involving  vacuum  pressure  in  some  cases,  has 
been  tried  with  considerable  success,  at  the  University  of 
Sheffield  and  elsewhere. 

Reference  may  also  be  made  to  articles  relating  to  the 
casting  of  glasshouse  pots  in  the  Journal  of  the  American 
Ceramic  Society,  2,  647,  659,  1919,  the  first  by  F.  H.  Riddle, 
and  the  second  by  J.  W.  Wright  and  D.  H.  Fuller  ;  also  to 
an  article  by  A.  V.  Bleininger,  /,  15,  1918. 

When  dried,  the  pots  are  set  in  a  small  auxiliary  furnace 
termed  a  "  pot  arch/'  where  they  are  annealed  and  then  heated 
to  whiteness,  after  which  they  are  removed  into  the  furnace 
for  use  in  melting  glass,  to  replace  old  pots  which  are  first 
withdrawn.  A  pot  is  left  in  the  furnace  empty  for  a  day 
or  two  to  regain  full  heat  before  raw  materials  are  charged 
into  it.  It  is  generally  first  glazed  inside  by  means  of  melted 
glass  from  another  pot.  The  cracking  of  pots  while  in  the 
furnace  is  very  troublesome.  Most  commonly  the  cracking 
takes  place  down  the  front  of  the  pot,  either  through  the 
lip  or  below  it,  and  the  mischief  is  probably  due,  as  Travers 
points  out,  to  insufficient  firing  in  the  pot  arch  and  subse- 
quent unequal  heating  in  the  glass  furnace.  The  pots 
removed  from  the  pot  arch  (where  the  temperature  probably 
seldom  reaches  1100°  C.)  are  subjected  in  the  furnace  to  a 
much  higher  temperature  (usually  1300°  to  1400°  C.),  which 
makes  at  least  the  surface  portions  semi- vitreous.  But 


300  SILICA   AND  THE  SILICATES 

under  ordinary  circumstances  the  front  of  the  pot  does  not 
reach  the  higher  temperature,  and  the  strains  thus  produced 
from  unequal  contraction  give  rise  to  a  tendency  to  crack. 
The  front  is  also  more  subject  to  corrosion.  All  this  could  be 
avoided  by  care  in  heating  the  pots  in  the  pot  arches,  and  by 
building  a  temporary  wall  in  front  of  the  pots  when  set  in 
the  furnace,  and  leaving  the  pots  undisturbed  several  days. 
An  instructive  paper  by  Dr.  M.  W.  Travers,  on  the  firing  of 
glasshouse  pots,  will  be  found  in  the  Journal  of  the  Society 
of  Glass  Technology,  2,  170,  1918. 

G.  W.  Wilson  (/.  Soc.  Glass  Technology,  2,  177,  1918) 
points  out  that  the  white  porcelain-like  layer  formed  round 
the  pots  or  blocks  is  often  packed  with  sillimanite  crystals. 
Sillimanite  is  only  developed  in  vitrified  pots,  and  it  lengthens 
the  life  of  the  pot  or  block  by  reducing  the  liability  to 
corrosion.  In  practice,  highly  aluminous  clays  should  be 
used,  but  the  vitrification  temperature  should  be  near  the 
furnace  temperature. 

For  very  basic  glasses,  plumbago  pots  have  been  found 
very  satisfactory.  They  are  made  of  mixtures  of  raw  fireclay 
and  graphite  (plumbago). 

lyieut.-Col.  C.  W.  Thomas  (in  /.  Soc.  Glass  Tech- 
nology, 4,  107,  1920)  suggests  making  covered  pots  in  two 
pieces,  the  upper  part  consisting  of  the  hood  and  adjoining 
portions  all  round  down  to  the  level  of  the  flat  lower  lip 
of  the  opening,  with  the  idea  of  improving  the  inside  of  the 
pot  by  beating  it,  when  hard  and  tough,  with  a  beater.  It 
is  proposed  to  fit  a  jointing  cushion  of  asbestos  rope  between 
the  two  parts,  each  of  which  is  grooved  to  hold  the  packing 
in  position.  The  asbestos  rope  is  said  to  make  an  effective 
gas-tight  and  dust-tight  joint.  This  device  is  not  patented. 

TANKS. 

For  the  production  of  glass  on  a  larger  scale,  the  ordinary 
pots  are  replaced  by  tanks,  more  particularly  for  bottle  glass 
and  other  common  glasses,  as  the  glass  melted  in  tanks  is 
much  more  liable  to  contamination  than  that  melted  in  pots  ; 


GLASS  AND  ENAMELS  301 

recent  improvements  have  been  effected  in  this  respect,  and 
owing  to  their  greater  capacity  tanks  may  be  preferred  in  the 
future  for  other  kinds  of  glass  as  well. 


Tank  furnaces  contain  only  one  tank,  of  rectangular 
shape,  commonly  about  18  to  24  ins.  deep,  and  30  to  100  ft. 
long,  and  are  economical  in  working.  The  bed  and  walls  of 


302  SILICA   AND   THE  SILICATES 

the  tank  are  made  ot  fireclay  blocks,  specially  selected  for 
resistance  to  corrosion.  These  furnaces  are  gas-fired.  The 
tank  has  a  centrally  placed  shallow  bridge,  which  prevents 
all  unmelted  material  from  passing  to  the  working  compart- 
ment. A  crown  or  arch  surmounts  the  tank,  and  openings 
at  the  working  end  enable  the  glass  workers  to  gather  the 
"  metal/'  as  the  melted  glass  is  called. 

B.  E.  Fisher,  an  American  glass  manufacturer  (/.  Soc. 
Glass  Technology,  3,  147,  1919),  strongly  advocates 
the  practice  of  heating  tank  furnaces  up  to  the  highest 
possible  temperature  before  charging  even  with  cullet,  this 
method  of  working  greatly  enhancing  the  durability  of  the 
tanks. 

It  is  noteworthy  that  tank  furnaces  were  first  introduced 
in  England.  They  were  afterwards  adopted  in  Germany, 
and  much  later  in  the  United  States. 

PRINCIPAL  TYPES  OF  GLASS. 

The  principal  types  of  glass  are  soda-lime  glass,  potash- 
lime  glass,  potash-lead  glass,  and  borosilicate  glasses. 

Soda- Lime  Glass,  or  white  glass,  is  often  called  "  crown  " 
glass,  though  that  name  originally  referred  to  a  particular 
method  of  manufacture.  The  composition  (as  previously 
mentioned)  sometimes  corresponds  nearly  to  2 ^SiO^  O'5CaO, 
0'5Na2O  (representing  about  72%  of  SiO2, 13%  of  CaO,  15%  of 
Na2O) .  Davidson  and  Turner (/.  Soc.  Glass  Technology,  3, 222, 
1920)  state  that  such  glasses  with  13  to  17  per  cent.  Na2O 
are  durable.  They  are  much  used  for  making  window-glass 
by  the  crown  or  disc  method,  and  by  the  cylinder  method. 

In  the  crown  process  the  workman  collects  on  his  blow- 
iron  (an  iron  tube  4  to  5  ft.  long)  about  10  to  14  Ib.  of  hot 
glass,  and  "  marvers  "  this  into  a  pear-shaped  lump  by 
spinning  the  glass  round  in  the  hollow  of  a  block  of  wood  ; 
sometimes  the  "  marvering  "  of  glass  is  done  on  a  "  marver  " 
consisting  of  a  thick  flat  plate  of  iron.  He  then  blows  the 
glass  out,  and  by  rapid  spinning  it  is  made  to  flatten  out  so 
that  its  diameter  becomes  many  times  greater  than  the 
distance  from  front  to  back.  Another  workman  then  fixes 


GLASS  AND  ENAMELS  303 

by  means  of  a  little  molten  glass  a  pontil  (or  pointel,  that  is, 
a  solid  iron  rod  of  about  the  same  length  as  the  blow-iron)  on 
the  middle  of  the  outer  curved  surface,  and  the  blow-iron 
is  disconnected  by  wetting  the  glass  close  to  the  blow-pipe, 
the  glass  breaking  off  and  leaving  an  aperture.  The  glass 
on  the  pontil  is  next  taken  to  a  "  flashing  "  furnace,  which 
has  a  large  round  opening  in  front ;  the  glass  soon  becomes 
softened,  and,  as  the  pontil  is  rotated  rapidly,  the  opening  in 
the  glass  gradually  enlarges  until  suddenly  the  glass  spreads 
out  into  a  large  flat  round  sheet  or  disc.  This  circular 
crown  table,  while  still  rotating,  is  brought  away  from  the 
furnace,  and  allowed  to  set,  and  is  then  conveyed  to  the 
annealing  oven,  the  pontil  being  removed,  leaving  its 
position  marked  by  the  "  bull's  eye  "  or  bullion. 

In  the  cylinder  method  for  making  what  is  called  Sheet 
Glass  (formerly  known  as  broad  glass)  the  workman  gathers 
glass  into  a  large  ball,  which  is  then  blown  out  and  widened 
by  rapidly  spinning  the  blow-iron  as  in  the  previous  method. 
But  in  this  case,  by  repeatedly  blowing  and  swinging  in 
turn,  the  shape  becomes  elongated  until  it  is  cylindrical. 
The  free  end  of  the  cylinder  is  reheated,  and  opened  out 
with  the  help  of  a  special  tool  until  the  diameter  at  the  open 
end  is  the  same  as  that  of  the  cylinder  in  general.  The 
end  attached  to  the  blow-iron  is  next  cracked  off,  which  may 
be  done  by  letting  the  glass  cool,  then  wrapping  a  thread  of 
hot  glass  round  it,  removing  the  thread,  and  quickly  applying 
a  cold  hook  at  the  end  of  an  iron  rod.  The  glass  cylinder 
is  taken  to  a  flattening  kiln,  being  first  cut  or  cracked  length- 
ways on  the  inside,  then  placed  further  in  the  kiln,  where 
the  heat  makes  it  uncurl  and  flatten  out  gradually.  As  the 
sheet  becomes  flattened,  it  is  levelled  out  with  a  flat  block  of 
charred  wood  called  a  "  polisher/'  Bach  sheet  when  levelled 
is  taken  away  to  the  annealing  oven.  This  method  gives 
larger  sheets  than  the  crown  glass  method,  but  cylinder  glass 
always  shows  some  surface  waviness  and  is  less  brilliant  than 
crown  glass.  The  superior  surface  of  crown  glass  is  doubt- 
less due  to  the  "  fire-polishing  "  it  gets  during  its  expansion. 
Glass  shades  are  also  blown  by  sheet  blowers. 


304  SILICA   AND  THE  SILICATES 

In  the  United  States  the  production  of  sheet  glass  by  the 
cylinder  method  was  successfully  accomplished  by  the 
application  of  mechanical  means,  the  cylinders  being  opened 
and  flattened  out  as  before.  Another  method — the  Colburn 
process — which  has  recently  been  practised  with  some  success 
in  America,  is  the  direct  drawing  out  of  the  molten  glass  into 
sheets  without  any  blowing  or  other  intermediate  operation. 

K.  H.  Bostock  has  suggested  (in  the  Journal  of  the  American 
Ceramic  Society,  3,  36,  1920)  that  possibly  an  extrusion 
method  might  be  devised  to  furnish  a  superior  product. 

Plate  Glass  is  another  variety  of  soda-lime  glass  used  for 
glazing  shop  windows  and  show  cases,  and  for  mirrors. 
More  care  is  taken  in  the  selection  of  materials  and  in  the 
manipulation  than  for  crown  or  sheet  glass.  The  melted 
glass  when  plain  and  free  from  seeds  is  either  ladled  out  into 
smaller  pots  for  casting,  or  (more  usually)  the  whole  pot  is 
removed  from  the  furnace,  and  after  skimming  the  glass  it  is 
taken  to  the  casting  table.  The  casting  table  consists  of  an 
iron  bench  with  level  top  and  a  raised  flange  running  along 
each  side  to  determine  the  thickness  of  the  glass  plate.  The 
hot  glass  is  poured  on  the  table,  and  a  heavy  metal  roller  is 
passed  along  to  roll  out  the  hot  glass  to  the  regulated  thick- 
ness. The  cast  plate  is  then  trimmed  at  the  ends,  and  when 
set  is  pushed  into  an  annealing  oven.  After  annealing,  the 
glass  plate  has  rather  wavy,  rough,  and  uneven  surfaces, 
and  has  to  be  ground  and  polished  on  both  sides  by  means  of 
a  grinding  machine.  It  is  then  ready  for  being  made  into 
mirrors  if  desired. 

Sometimes  the  molten  glass  is  passed  between  two  or 
more  parallel  rollers  to  make  plate  glass. 

Wired  glass,  or  strengthened  plate,  is  produced  by 
causing  a  network  of  specially  suitable  wire  to  be  embedded 
in  the  soft  glass  during  the  rolling.  Plate  glass  may  also  be 
strengthened  by  connecting  two  plates  with  an  intervening 
celluloid  film. 

Mirrors  are  made  by  laying  tinfoil  over  a  smooth  hori- 
zontal table,  pouring  mercury  (quicksilver)  on  the  tinfoil 
and  distributing  it  evenly,  then  carefully  sliding  a  glass  plate 


GLASS  AND  ENAMELS  305 

over  the  mercury  without  touching  the  tinfoil  with  it, 
applying  weights  on  the  plate  so  that  the  latter  is  brought 
into  contact  with  the  tinfoil  and  the  excess  of  mercury  is 
squeezed  out  gradually.  Another  method  of  silvering  glass 
plates  is  by  the  reduction  of  silver  nitrate  solution  to  which 
has  been  added  excess  of  ammonia  and  also  some  reducing 
agent  such  as  grape  sugar,  oil  of  cloves,  etc.  The  glass  plate 
is  well  cleaned,  placed  horizontally,  and  the  silvering  solution 
is  poured  over  the  surface,  the  temperature  conditions  being 
kept  favourable.  When  a  sufficient  covering  of  silver  has 
been  formed,  the  liquid  is  poured  off,  the  plate  is  dried,  and 
the  silver  film  is  protected  by  a  coating  of  hard  varnish. 

Pressed  Glass  is  practically  a  crown  glass  in  which  part 
of  the  lime  is  replaced  by  barium  oxide  (baryta),  the  latter 
helping  to  give  the  pressed  ware  a  good  surface.  Seam 
marks  are  left  on  the  pressed  glass  articles.  Pressed  glass- 
ware is  made  by  dropping  hot  glass  into  a  two-piece  iron 
mould  fixed  in  a  lever  press,  so  that  by  bringing  down  a  lever 
arm  the  plunger  presses  the  hot  glass  into  shape.  The 
plunger  is  then  released,  and  the  pressed  article  turned  out  of 
the  mould,  the  article  being  removed  for  finishing  operations 
before  being  annealed. 

Bohemian  Glass  has  potash  instead  of  soda,  along  with 
lime  and  silica,  and  a  little  alumina.  It  forms  a  hard, 
brilliant  glass,  much  used  for  tableware,  mirror  glass,  and 
chemical  glassware,  and  is  specially  suitable  for  enamelled 
glassware. 

Bottle  Glass  is  a  variety  of  lime-soda  glass  containing  a 
distinctly  larger  percentage  of  alumina  at  the  expense  of  the 
silica.  Some  of  the  soda  may  be  replaced  by  other  bases, 
such  as  magnesia.  It  is  more  complex  in  composition  than 
other  ordinary  glasses.  It  is  very  hard,  and  more  resistant 
to  chemical  action,  as  well  as  more  difficult  to  melt,  than 
ordinary  crown  glass.  It  is  melted  in  tank  furnaces. 
Common  bottle  glass  is  coloured,  usually  greenish,  according 
to  the  amount  of  iron  and  other  colouring  substances  present 
in  it.  Common  sand  (which  would  not  be  pure  enough  for 
other  varieties)  is  used  in  its  manufacture,  as  well  as  other 
c.  20 


3o6  SILICA   AND   THE  SILICATES 

materials  of  common  quality.  Even  basalt,  granite,  phono- 
lite,  and  other  igneous  rocks  have  been  used  in  its  preparation, 
and  also  artificial  materials  such  as  blast-furnace  slag. 

Mixtures  containing  equal  parts  of  decomposed  basaltic 
earth  and  sand,  as  well  as  other  mixtures  including  basalt, 
were  found  to  give  a  dark-coloured,  hard,  tough  glass  highly 
resistant  to  corrosive  liquids.  Difficulty  arose  in  getting 
materials  sufficiently  uniform.  Basaltic  earth  alone  was  also 
used. 

Nearly  fifty  years  ago  in  this  country  a  process  was 
evolved  and  patented  for  preparing  glass  from  a  mixture  of 
100  parts  of  Welsh  or  South  Staffordshire  iron  slag,  65  of 
common  ferruginous  sand  (yellow  or  red),  and  10  of  common 
sodium  sulphate  (salt  cake).  The  appliances  for  the  manu- 
facture of  glass  from  slag,  involving  the  utilization  also  of  the 
heat  of  the  slag,  were  described  at  a  meeting  of  the  Iron  and 
Steel  Institute  at  Leeds  in  1876  (see  the  Institute's  Jo urnal  for 
1876,  p.  453),  and  many  specimens  were  then  exhibited,  the 
colour  of  which  was  stated  to  be  little  if  any  darker  than  much 
of  the  rough  glass  plate  at  that  time  used  for  skylights.  See 
also  Bashley  Britten's  patent  No.  3750,  November  19,  1873. 
There  may  have  been  good  reasons  for  hesitating  about 
adopting  such  a  process  at  that  period,  but  in  the  light  of 
recent  experience,  and  especially  with  modern  bottle-making 
machinery  available,  the  idea  seems  now  to  merit  the  serious 
consideration  of  practical  glass  workers. 

It  may  be  of  interest  to  give  the  following  quotation  from 
the  fourth  volume  (Supplement),  p.  817,  of  lire's  "  Dictionary 
of  Arts,  Manufactures,  and  Mines,  1878,"  which  was  edited 
by  the  late  Robert  Hunt :  "  We  learn  from  Mr.  Britten  that 
the  extensive  experiments  which  have  been  carried  on  during 
the  years  1876-7  have  proved  perfectly  successful,  and  that 
under  the  title  of  '  Britten's  Patent  Glass  Company,'  for 
which  Mr.  Herbert  Canning  is  the  Secretary,  large  works  are 
being  built  at  Finedon,  in  Northamptonshire,  where  in  a  few 
months  they  will  be  ready  to  manufacture  large  quantities 
of  glass  bottles."  I  am  not  able  to  supply  any  later  reference 
to  this  venture. 


GLASS  AND  ENAMELS  307 

The  shaping  of  bottles  is  at  present  done  mostly  by  means 
of  automatic  or  semi-automatic  machinery,  which  originated 
chiefly  in  the  United  States.  Common  bottles  are  extensively 
made  in  this  way,  but  sometimes  a  hand  tool  finishes  the 
neck  after  the  mould  has  formed  the  body.  Modern  bottle- 
making  machines  have  been  brought  to  such  a  degree  of 
perfection  that  they  gather  the  metal,  effect  the  shaping, 
and  complete  the  bottle,  so  that  hand  labour  is  practically 
superseded  in  the  actual  making  of  the  bottles ;  travelling 
arms  are  made  to  cool  the  mould  forms  by  dipping  them  into 
water,  the  moulds  then  open  to  receive  the  hot  metal  and 
afterwards  close,  air  pressure  is  applied  within  the  mould, 
and  the  bottle  is  then  delivered  on  to  a  travelling  belt  which 
takes  it  away  to  be  annealed.  The  output  of  some  of  these 
bottle-making  machines  is  enormous. 

It  may  be  mentioned  that  in  this  country  the  Hartford- 
Fairmont  patents  have  been  introduced  into  a  number  of 
works  by  the  British  Hartford-Fairmont  Syndicate,  lytd., 
London ;  these  include  the  Hartford-Fairmont  Feeder  and 
the  O'Neill  bottle-making  machines.  The  Millar,  Lynch,  and 
Owens  are  other  well-known  machines.  The  Owens  machine 
is  capable  of  producing  700  bottles  in  an  hour. 

For  working  with  automatic  or  semi-automatic  machines, 
it  is  found  advantageous  to  have  the  glass  less  viscous  than 
for  ordinary  blowing,  and  this  is  ensured  by  a  slight  increase 
in  the  percentage  of  soda  in  soda-lime  glasses,  the  best  pro- 
portions of  soda  being  15  to  17  per  cent.,  according  to 
Davidson  and  Turner's  researches. 


FUNT  GLASS  OR  CRYSTAL. 

Flint  glass  or  crystal  is  essentially  a  silicate  of  potash  and 
lead  oxide,  the  density  increasing  with  the  propoition  of  lead, 
which  latter  is  increased  at  the  expense  of  the  silica  and  potash 
jointly.  It  is  the  greater  fusibility  due  to  the  lead  which 
enables  extra  silica  to  be  taken  up,  so  that  in  some  cases  the 
composition  of  flint  glass  approximates  to  3'oSiO2,  o^PbO, 
o*5K2O.  Good  ordinary  flint  glass  is  colourless,  but  excess 


308  SILICA   AND  THE  SILICATES 

of  lead  imparts  a  yellowish  tinge.  All  or  part  of  the  potash 
may  be  replaced  by  soda,  but  the  resulting  glass  has  not  the 
same  brilliance.  According  to  Hodkin  and  West  (J.  Soc. 
Glass  Technology,  #,  124, 1920),  2  parts  borax  per  100  parts 
sand  in  the  batch  increases  the  durability  of  lead  glasses,  as 
does  also  equivalent  proportions  of  soda  and  potash  instead 
of  either  alone.  Most  English  cut  glass  is  made  from  the 
best  crystal.  Bismuth  oxide  might  be  substituted  for  lead 
oxide  in  the  manufacture  of  glass,  giving  a  very  similar 
product,  but  it  is  much  too  expensive  for  general  use. 

The  proportions  of  the  ingredients  actually  used  for 
making  flint  glass  vary  considerably.  Nitre  (saltpetre), 
manganese  dioxide,  and  white  arsenic  (arsenic  trioxide)  are 
added  in  small  quantities.  The  materials  intimately  mixed 
together  are  charged  into  the  covered  pots,  previously  heated 
to  a  white  heat,  so  as  to  fill  them.  When  all  is  melted  the 
bulk  is  much  less,  and  a  further  charge  is  added,  this  being 
repeated  until  the  vessels  become  full  of  melted  glass.  Even 
when  melted  the  glass  only  loses  its  opacity  gradually,  and  it 
only  becomes  transparent  after  the  removal  of  a  white  porous 
scum  which  rises  through  the  glass,  and  is  known  as  glass- 
gall  or  sandiver.  This  scum  apparently  consists  of  certain 
salts  present  in  alkalies  ;  these  have  a  lower  specific  gravity 
than  the  glass  and  have  only  slight  affinity  for  silica.  Another 
substance  called  sandiver  is  sometimes  found  at  the  bottoms 
of  pots,  and  is  shifted  when  the  glass  is  worked  off  ;  this  is 
quite  different  from  the  scum  previously  referred  to,  and  is  a 
vitrified  mass  of  earthy  and  other  impurities.  Glass-gall 
is  very  volatile  at  high  temperatures,  and  the  resulting  dense 
vapour  acts  corrosively  on  the  pots.  When  vitrification 
becomes  complete,  the  glass  is  in  too  fluid  a  condition  for 
shaping,  so  its  temperature  is  lowered  by  stopping  the  draught 
in  the  part  of  the  furnace  where  the  pot  is,  when  the  glass 
loses  its  perfect  fluidity  and  becomes  soft  and  tenacious. 
Below  redness  glass  becomes  rigid,  brittle,  and  transparent. 
Glass  may  remain  a  long  time  in  the  melted  state  without 
changing  its  qualities,  that  is,  the  working  temperature 
has  a  considerable  range.  The  blow-iron  and  pontil  are 


GLASS  AND  ENAMELS  309 

used,  in  the  manner  already  referred  to,  for  collecting  glass 
and  blowing  it  into  various  shapes. 

Lead  Silicates  are  more  fusible  as  the  proportion  of  lead 
oxide  increases.  The  metasilicate  (SiO2.PbO  or  PbSiO3) 
melts  at  a  bright-red  heat,  and  is  itself  yellowish. 

Rocaille  Flux,  strass  metal,  or  diamond  paste,  is  made 
by  fusing  together  100  parts  sand  and  66  parts  red  lead. 
It  is  used  chiefly  for  making  soft  enamels  (by  addition  of 
borax)  and  imitation  gems.  The  original  Rocaille  flux  was 
a  silicate  of  lead  containing  lead  oxide  (or  red  lead)  in  the 
proportion  of  3  parts  to  i  part  of  sand  ;  this  has  a  yellow 
colour. 


DEVITRIFICATION. 

When  glass  is  kept  for  a  time  at  the  freezing  point  (or 
solidification  point)  of  any  of  its  constituent  compounds,  such 
compound  or  compounds  will  tend  to  crystallize  out  from 
solution,  and  glass  which  thus  becomes  cryptocrystalline  is 
said  to  be  devitrified.  Reaumur  in  1727  noticed  that  glass 
heated  for  a  long  time  at  its  softening  temperature,  but 
without  melting,  became  dull,  opaque,  and  milky  white, 
and  such  devitrified  glass  was  called  Reaumur's  porcelain. 

Glasses  rich  in  lime  and  alumina  readily  devitrify,  whilst 
glasses  rich  in  lead  or  alkali  can  be  maintained  at  the  critical 
temperature  for  a  long  time  before  devitrification  takes 
place.  Smaller  proportions  of  alumina,  as  well  as  boric 
oxide  and  oxides  of  titanium,  zirconium,  tin,  thorium, 
arsenic,  and  antimony  tend  to  prevent  devitrification. 
Devitrification  is  sometimes  intentionally  promoted  for 
decorative  purposes,  as  in  "  ambitty  sheet." 

In  the  widest  sense  of  the  term,  devitrification  means  the 
separation  from  glass  of  any  constituent  in  either  crystalline 
or  amorphous  condition.  Some  writers  regard  it  as  crystal- 
lization, whether  evidently  so  or  (as  is  more  usually  the  case) 
in  an  incipient  stage.  Others  consider  as  devitrification 
only  visible  aggregates,  whether  crystalline  or  amorphous, 
as  distinguished  from  opalization,  which  results  from 


3io  SILICA   AND  THE  SILICATES 

separation  of  very  finely  divided  particles,  either  crystalline 
or  amorphous. 

Separations  from  glass  fall  into  four  classes  : 

(1)  Separation  of  easily  visible  crystalline  particles,  with 
no  milkiness  or  cloudiness  in  the  glass. 

(2)  Separation  of  easily  visible  non-crystalline  particles. 

(3)  Separation  of  minute  crystalline  particles  giving  the 
glass  a  milky  or  opal  effect. 

(4)  Separation  of  minute  non-crystalline  particles  with 
accompanying  opal  effect. 

C.  J.  Peddle  (/.  Soc.  Glass  Technology,  4,  3,  1920)  suggests 
for  these  four  varieties  the  names  crystalline  devitrification, 
amorphous  devitrification,  crystalline  opalization,  and  amor- 
phous opalization  respectively.  It  is  implied  that  in  each 
case  there  was  at  one  time  total  solution,  followed  by  separa- 
tion. Some  "  opal "  glasses  instead  of  being  clear,  are 
suspensions  (not  solutions),  when  gathered  at  founding 
temperatures. 

A  disadvantage  of  Peddle's  proposal  as  it  stands  is  that 
it  provides  no  satisfactory  term  to  cover  the  four  varieties, 
and,  as  all  the  varieties  represent  important  departures 
from  the  normal  structure  of  glass,  devitrification  seems  a 
very  appropriate  comprehensive  term  to  use  in  the  widest 
sense  as  defined  above.  The  first  and  second  classes  of 
separations  might,  for  want  of  better  terms,  be  referred  to  as 
crystalline  and  amorphous  segregations  respectively,  when  it 
is  desired  to  emphasize  the  distinction  from  opalization. 

The  commonest  crystalline  deposit  in  glass  seems  to  be 
silica,  which  may  separate  from  various  types  of  glass, 
including  lime-soda  glasses ;  it  is  mostly  in  the  form  of 
cristobalite,  but  with  some  tridymite.  Calcium  meta- 
silicate  (CaSiO3)  may  separate  from  ordinary  lime-soda 
glasses.  From  glasses  very  rich  in  lead  may  be  deposited 
lead  metasilicate  (PbSiO3).  All  these  may  develop  as 
spherulites — more  or  less  globular  masses  consisting  of 
radiating  microscopic  crystals  with  interstitial  glassy  material. 
In  some  special  glasses  crystals  of  barium  disilicate  (BaSi2O5) 
may  separate. 


GLASS  AND  ENAMELS  311 

Closely  revSembling  devitrification  "  stones  "  in  general 
appearance  are  pot  stones,  batch  stones,  and  crown  drops. 
Pot  stones  are  derived  from  pots,  and  consist  entirely  of 
crystals  of  sillimanite  with  a  little  interstitial  glass.  Batch 
stones  consist  entirely  of  silica,  often  a  quartz  core  surrounded 
by  cristobalite  (or  cristobalite  and  tridymite),  or  occasionally 
entirely  cristobalite  ;  they  represent  undissolved  silica  of 
the  batch,  but  considerably  altered.  Crown  drops  are  due  to 
the  corrosive  action  of  vapours  on  bricks  of  the  crown,  and 
consist  of  crystals  of  tridymite  coarser  than  the  fine-grained 
cristobalite  of  batch  stones. 

Reference  may  be  made  to  two  papers  in  the  Journal  of 
the  American  Ceramic  Society  by  N.  Iy.  Bowen,  on  "  Identifi- 
cation of  Stones  in  Glass  "  (/,  594,  1918),  and  "  Devitrifica- 
tion of  Glass  "  (2,  261,  1919). 

SOME  SPECIAL  KINDS  OF  GLASS. 

Aventurine  (or  avanturine)  is  a  golden  yellow  glass 
with  minute  yellowish  spangles,  the  reflections  from  which 
give  the  characteristic  appearance.  It  is  produced  by  using 
excess  of  copper  along  with  strong  reducing  agents  in  the 
glass ;  partial  reduction  of  the  copper  taking  place  within 
the  glass.  Chromium  aventurine  is  a  somewhat  similar 
product,  obtained  by  the  use  of  excess  of  chromium  com- 
pounds in  the  presence  of  reducing  agents,  and  consisting  of 
a  translucent  green  glass  filled  with  minute  spangles.  In 
both  cases  slow  cooling  of  the  melted  glass  is  necessary  to 
aid  crystallization.  A  remarkable  effect  is  also  obtained  by 
rolling  or  marvering  some  dark-coloured  glass  upon  a  thin 
layer  of  flaked  mica,  then  making  another  gathering  or  coating 
of  clear  crystal  glass,  and  finally  blowing  the  whole  so  as  to 
form  an  ornament  or  vase,  when  the  mica  flakes  give  a  silvery 
reflection  against  the  dark  background. 

Gold  Ruby  Glass. — Gold  forms  very  unstable  colour- 
less compounds  with  glass,  and  the  gold  can  only  be  kept  in 
solution  by  cooling  suddenly.  When  cooled  slowly,  or  when 
the  quickly-cooled  colourless  glass  is  reheated  so  as  to  soften 


312  SILICA    AND   THE  SILICATES 

it,  metallic  gold  is  precipitated  as  minute  particles  dispersed 
all  through  the  glass,  and  the  glass  acquires  a  fine  ruby  red 
colour,  due  to  the  reflection  by  these  minute  particles  of 
light  of  shorter  wave  length  than  red.  A  very  small  quantity 
of  gold  is  required,  the  solubility  of  gold  in  glass  not  ex- 
ceeding O'Oi  per  cent. 

Red  Glass  may  also  be  obtained  by  the  use  of  copper 
along  with  a  reducing  agent ;  the  copper  can  be  thus  pre- 
cipitated as  minute  metallic  particles,  giving  a  ruby  red 
colour  in  the  same  way  as  with  the  gold  particles  referred  to 
above.  When  completely  oxidized  copper  forms  soluble 
silicates,  which  give  a  strong  green  or  blue  colour,  depending 
on  the  composition  of  the  glass. 

Silver  acts  in  the  same  way  as  gold,  but  the  particles 
transmit  yellow  light  in  this  case.  The  silver  is  readily 
absorbed  by  the  glass,  and  sheet  glass  is  coloured  yellow  by 
penetration  in  this  way.  On  painting  glass  with  any  silver 
salt,  and  heating  to  about  400°  C.,  the  glass  absorbs  some 
silver,  and  on  raising  the  temperature  to  about  800°  C. 
the  silver  is  precipitated,  producing  what  is  technically  termed 
silver  stain.  Potash-lime  glass  with  low  silica  gives  the 
richest  effect,  but  the  silver  is  not  easily  absorbed  by  hard 
glasses. 

Carbon  colours  glass  yellow  to  brown,  but  in  lead  glasses 
the  lead  would  be  reduced  as  metal. 

Sulphur  produces  a  greenish  yellow  coloration  (except 
in  lead  glasses)  due  to  formation  of  coloured  sulphides  of 
alkali  and  calcium.  Addition  of  7  to  10  per  cent,  of  sulphur 
gives  a  black  glass. 

Selenium  produces  a  pale  rose  coloration,  especially 
when  calcium  is  replaced  by  barium.  The  finest  rose 
colour  is  got  with  potash  glasses.  According  to  O.  N. 
Witt,  alkali-lime  glasses  take  up  at  most  0*06  per  cent, 
selenium,  the  excess  being  volatilized,  and  it  is  better  to 
introduce  the  selenium  as  a  selenite  or  a  selenate. 

Kirkpatrick  and  Roberts  (J.  American  Ceramic  Soc., 
2,  895,  1919)  state  that  selenium  gives  a  very  pure  red  colour 
to  some  glasses,  but  the  red  colours  from  other  colouring 


GLASS  AND  ENAMELS  313 

agents  transmit  light  from  other  parts  of  the  spectrum. 
After  the  selenium  becomes  incorporated  in  the  glass  (pro- 
bably in  colloidal  suspension)  it  is  retained  permanently  if 
the  temperature  does  not  exceed  1400°  C.  Good  red  colours 
were  produced  in  a  zinc-alkali  glass  with  0*8  per  cent,  each 
of  borax,  cadmium  sulphide,  and  selenium,  and  in  a  plate 
glass  by  using  0*65  per  cent,  selenium  and  0*85  per  cent, 
cadmium  sulphide.  The  two  most  essential  steps  are : 
(i)  to  prevent  escape  of  selenium  vapour,  and  (2)  to  allow 
the  glass  to  cool  long  enough  immediately  after  gathering 
(usually  30  to  60  seconds). 

Uranium  Oxide  gives  a  peculiar  fluorescent  yellow 
coloui.  The  fluorescence  is  stronger  in  potash-lime  glasses 
than  in  soda-lime  glasses,  and  is  not  noticeable  in  lead 
glasses.  Glass  coloured  by  uranium  oxide  or  sulphur  is 
often  used  when  it  is  desired  to  exclude  the  chemically  active 
(actinic)  light  rays. 

Antimony  Oxide,  combined  with  lead  oxide  to  form  lead 
antimoniate,  gives  an  opaque  yellow  colour  with  only  lead 
glasses. 

Manganese  Oxide  gives  a  reddish  violet  colour  to  lead 
glass  or  soda-lime  glass,  but  an  amethyst  tint  to  potash-lime 
glass.  The  tint  is  modified  or  even  discharged  altogether 
when  reducing  agents  are  present.  Glass  containing  manga- 
nese can  be  coloured  purple  even  by  exposure  to  actinic 
light,  and  the  colour  of  such  purple  glass  on  heating  it  to  the 
softening  point  can  be  made  to  disappear. 

Cobalt  Oxide  strongly  colours  all  glasses  blue,  with  o'i 
per  cent.  Even  0*01  per  cent,  produces  a  pale  blue. 

Nickel  Oxide  also  strongly  colours  glass  reddish  to 
purple,  but  its  action  is  somewhat  uncertain,  so  it  is  not 
much  used  for  colouring  purposes,  though  it  is  sometimes 
employed  instead  of  manganese  dioxide  as  a  decolorizer. 

Chromium  Oxide  produces  green  tints,  but  it  is  only 
slightly  soluble  in  glass,  and  any  excess  over  4  to  5  per  cent, 
separates  out  on  cooling,  producing  an  opaque  green  glass 

By  various  combinations  of  these  colouring  agents  a 
very  wide  range  of  tints  can  be  obtained.  The  tint  of  glass 


314  SILICA    AND   THE   SILICATES 

may  vary  if  the  thickness  of  the  glass  is  not  uniform,  as  is 
the  case  with  coloured  solutions.  Black  glass  is  obtained 
by  the  use  of  mixtures  of  cobalt  oxide,  iron  oxide,  and  some 
other  oxide  (as  manganese  oxide  or  nickel  oxide)  ;  a  very 
fine  black  is  produced  by  iridium  oxide. 

A  practically  opaque  glass  is  the  subject  of  a  patent 
by  the  Corning  Glass  Works,  etc.  (pat.  127,586,  May  28, 
1919)  ;  it  is  very  transparent  for  ultra-violet  rays.  The 
preferred  composition  is  given  as  50  per  cent,  of  SiO^, 
16  of  K2O,  25  of  BaO,  and  9  of  NiO. 

White  or  opal  glasses  are  produced  by  including  in  the 
batch  materials  either  tin  oxide,  calcium  phosphate,  calcium 
fluoride,  cryolite,  alumina,  zinc  oxide,  talc,  arsenious  oxide 
(white  arsenic),  or  more  than  one  of  these.  Tin  oxide  is 
now  much  too  expensive  for  general  use. 

Sprechsaal  (52,  30,  265,  1919)  gives  two  typical  batches 
for  opal  glass  without  cryolite,  with  the  use  of  bone-ash  in 
one  case,  and  a  mixture  of  felspar  and  fluorspar  in  the  other, 
as  opacifying  agents  : 

Sand.    Potash.  Soda-ash.   Limespar.   Bone-ash.   Borax.  Saltpetre.   Fluorspar.  Felspar. 
I.    lOO'O       IO'O          24/5  l8'O  22'O  2'O  2'O  

II.  loo-o       2-5       27-5        —  i '5        —  ii'o       27*5 

Alabaster  glass  is  semi-opaque,  and  does  not  materially 
alter  the  colour  of  the  light  it  transmits,  whereas  in  the  case 
of  opal  glass,  which  is  also  semi-opaque,  the  transmitted 
light  acquires  an  opal  or  fiery  colour.  The  most  desirable 
alabaster  glasses  are  produced  by  aluminous  substances 
with  fluorides  and  chlorides  or  sulphates.  Other  substances 
used  for  the  same  purpose  are  calcium  phosphate,  magnesium 
silicate,  tin  oxide,  arsenious  oxide,  zirconia  (Ger.  pat.  189,134), 
and  titanium  oxide  (Ger.  pat.  18,708).  Alabaster  effects 
may  also  be  obtained  in  high  silica  glasses  through 
devitrification. 

The  earlier  white  glasses  were  chiefly  alabaster  glasses, 
due  to  the  effect  of  impurities  in  the  form  of  chlorides  and 
sulphates  present  in  the  potash  used,  which  caused  the 
precipitation  of  the  colloids  in  the  glass.  Reference  may  be 


GLASS  AND  ENAMELS  315 

made  to  a  comprehensive  paper  by  A.  Silverman  on  "  Ala- 
baster Glass  "  in  the  Journal  of  the  American  Ceramic  Society, 
i,  247,  1918,  which  includes  a  copious  bibliography,  etc. 

Sometimes  coloured  glass  has  not  the  colour  distributed 
throughout.  Such  glass  is  made  by  the  blower  dipping  the 
end  of  his  pipe  first  in  coloured  glass,  then  in  colourless  glass, 
or  vice  versa ;  or  he  may  take  up  first  colourless  glass,  then 
coloured  glass,  and  then  colourless  glass  again  ;  in  any  case 
he  proceeds  to  blow  the  glass  into  a  cylinder,  which  can  then 
be  cut  and  flattened  out  like  sheet  glass. 

As  glazes  are  merely  glasses  spread  in  thin  layers,  there 
seems  no  valid  reason  why  the  suggestions  thrown  out  in 
connection  with  crystalline  ceramic  glazes  (in  Section  IV.) 
should  not  (with  the  aid  of  proper  appliances)  be  more  or 
less  applicable  to  glass  also.  In  that  case,  it  should  be  a 
comparatively  easy  matter  to  colour  the  surface  of  glass  red 
with  copper  or  gold,  yellow  with  silver,  etc.  (See  p.  225.) 

GI,ASS  PAINTING  OR  STAINING. 

Glass  painting  or  staining  implies  the  art  of  painting  on 
glass  (either  colourless  or  already  coloured)  with  vitrified  or 
vitrifiable  colours,  which  are  afterwards  fixed  to  the  surface 
of  the  glass  by  firing  in  a  muffle.  In  some  cases  a  flux  or 
glassy  composition  has  to  be  mixed  with  the  colouring  base 
to  enable  it  to  be  fixed  to  the  glass.  This  flux  is  sometimes 
simply  mixed  with  the  colouring  substance  before  firing  it 
on  the  glass,  but  in  other  cases  it  is  vitrified  by  previous 
melting  with  the  colouring  substance,  and  then  ground 
before  putting  it  on  the  glass.  The  effect  is  practically  the 
same  as  laying  finely  powdered  coloured  glasses  on  the 
surface  of  the  glass  to  be  painted,  and  then  firing.  The 
various  substances  should  of  course  be  insoluble  in  water, 
and  they  should  have  (after  firing)  approximately  the  same 
coefficient  of  expansion  as  the  glass  to  which  they  are  to  be 
fixed  ;  the  fusibility  should  be  greater  than  that  of  the  glass, 
which  is  generally  ensured  by  the  addition  of  the  colouring 
substance. 


316  SILICA    AND    THE   SILICATES 

The  flux  may  be  simply  a  combination  of  silica  (i  part) 
and  lead  oxide  (3  parts),  a  combination  long  known  as 
Rocaille  flux ;  more  generally  the  flux  contains  also  some 
borax,  and  sometimes  nitre  as  well,  the  ingredients  being 
present  in  various  proportions.  Flint  glass,  mixed  with 
potash  (pearl-ash)  and  a  little  borax,  is  sometimes  used. 
For  some  colours  lead  oxide  is  undesirable,  and  borax  may 
then  be  used. 

OPTIC AI,  GLASS. 

Optical  glass  is  not  circumscribed  by  any  narrow  limits 
as  regards  chemical  composition,  but  may  belong  to  any  of 
the  main  groups  of  glasses,  such  as  the  normal  or  modified 
crown  or  flint  glasses,  borosilicate  glasses,  etc.  In  view  of 
the  special  importance  of  optical  glass,  it  may  be  well  to 
briefly  review  the  characteristics  of  greatest  value  in  glasses 
intended  for  optical  purposes.  In  general,  optical  glasses 
may  be  said  to  be  superior  to  ordinary  glass  as  regards  such 
properties  as  transparency  and  colour,  hardness,  and  dura- 
bility. The  optical  properties,  which  determine  the  refrac- 
tion and  dispersion  of  the  rays  of  light,  vary  widely  according 
to  the  composition  of  the  glass  and  the  treatment  it  receives 
in  the  course  of  manufacture. 

For  many  lenses  of  a  cheap  type,  and  usually  small  in 
size,  the  stipulation  as  to  superior  quality  applies  only  in 
a  relative  sense.  Ordinary  plate  glass,  for  instance,  is  largely 
used  for  making  cheap  lenses,  when  it  is  of  reasonably  good 
quality. 

Transparency  and  colour  are  closely  connected,  for  the 
degree  of  transparency  may  (and  often  does)  vary  for 
different  coloured  rays.  At  a  casual  glance,  common 
window  glass  seems  colourless,  but  if  viewed  through  a  con- 
siderable thickness  (several  inches),  as  by  looking  through 
a  piece  edgeways,  it  appears  of  a  sea-green  tint.  Even  the 
very  best  optical  glass  made  from  exceptionally  pure  materials 
seems  distinctly  coloured  when  light  passes  through  a  con- 
siderable thickness  of  it ;  this  of  course  is  due  to  absorption 
of  certain  light  rays  as  they  pass  through  the  glass.  Common 


GLASS  AND  ENAMELS  317 

glasses  owe  their  greenish  colour  to  the  presence  of  impurity, 
mainly  ferrous  oxide,  and  this  is  also  true  (in  a  lesser  degree) 
of  the  better  glasses.  It  is  noteworthy  in  this  connection 
that  the  same  impurity  present  in  the  same  proportion  may 
produce  a  much  more  pronounced  effect  in  one  kind  of  glass 
than  in  another.  Thus,  the  effect  of  the  impurity  which 
would  produce  a  distinct  greenish  tint  in  a  dense  barium 
glass  would  scarcely  be  noticeable  in  a  hard  crown  glass. 
To  avoid  the  greenish  coloration  which  is  popularly  associated 
with  glass  of  poor  quality,  glass  makers  use  "  decolorizers," 
that  is,  substances  which  alone  would  produce  a  reddish 
tinge,  the  effect  being  to  neutralize  the  greenish  colour.  But 
this  remedy  is  very  inappropriate  in  the  case  of  optical  glass, 
because  it  involves  further  absorption  of  light  rays,  with 
consequent  loss  of  light.  As  it  is  usually  necessary  for 
optical  purposes  to  have  as  much  light  as  possible  transmitted, 
it  is  important  to  avoid  colour  as  far  as  practicable  by 
excluding  colouring  impurities.  Apart  from  all  this,  the 
colour  of  a  glass  may  be  intimately  associated  with  important 
optical  properties,  arising  from  unequal  dispersion  in 
different  parts  of  the  spectrum. 

Transparency  of  glass  is  diminished  by  the  presence  of 
foreign  bodies,  and  these  should  be  cut  out  during  the 
selection  of  the  glass,  and  their  presence  obviated,  when 
practicable,  during  the  process  of  manufacture.  More 
trouble  is  caused  by  foreign  bodies  which  arise  in  the  glass 
itself,  as  bubbles  or  "  seed,"  and  as  devitrification  products. 
Bubbles  or  "  seed  "  are  liable  to  appear  in  all  glass,  and  may 
be  derived  in  the  first  place  from  air  in  the  powdered  batch, 
and  afterwards  from  gases — chiefly  carbon  dioxide  and 
nitrogen  oxides — liberated  by  decomposition  of  carbonates 
and  nitrates  among  the  raw  materials.  By  adjustments 
to  suit  the  furnace  and  the  particular  method  of  working, 
a  manufacturer  can  generally  arrange  to  use  a  mixture  which 
gives  mostly  large  bubbles,  by  which  the  smaller  ones  are 
swept  out  of  the  melted  glass.  But  in  the  manufacture  of 
optical  glass  the  manufacturer  has  much  less  latitude,  since 
it  is  necessary,  in  order  that  the  glass  shall  have  the  stipulated 


3i8  SILICA    AND   THE   SILICATES 

optical  properties,  to  produce  a  glass  of  definite  chemical 
composition.  In  some  cases  the  glass  cannot  be  produced 
quite  free  from  small  bubbles,  but  as  these  do  not  involve  the 
loss  in  transmission  of  more  than  0*02  per  cent,  of  the  light, 
it  is  no  serious  disadvantage  to  the  lens.  Devitrification 
has  already  been  considered.  Generally  it  is  possible  to 
avoid  this  difficulty  by  cooling  the  glass  quickly  through 
the  dangerous  zone  of  temperature,  which  for  most  glasses 
is  in  the  neighbourhood  of  a  dull-red  heat. 

The  most  serious  interference  with  transparency  comes 
from  the  "  striae/'  or  veins,  which  occur  in  all  ordinary 
glass,  and  which  have  to  be  removed  for  the  best  optical 
glass.  They  arise  from  irregularities  in  the  mixing  of 
portions  of  melted  material  of  different  densities  and  different 
refractive  indices,  and  are  variously  disposed  in  streaks, 
streams,  layers,  etc.  Careful  stirring  might  naturally  be 
expected  to  result  in  uniform  mixing,  but  the  matter  is 
greatly  complicated  by  the  corrosive  action  of  the  molten 
glass  on  the  material  (usually  fireclay)  of  the  pot ;  the  product 
of  this  action  is  very  viscous  and  mixes  with  the  other  glass 
slowly,  and  while  it  is  being  incorporated  by  stirring,  fresh 
striae  would  be  forming.  Hence  the  stirrer  has  to  be  kept 
away  from  the  walls  of  the  pot,  and  only  a  portion  of  the 
contents  can  be  freed  from  striae.  In  plate  glass  and  sheet 
glass  (window  glass)  striae  are  numerous  and  are  arranged 
in  fairly  even  layers  parallel  to  the  surface  ;  they  appear 
clearly  when  the  glass  is  viewed  through  a  polished  edge. 
In  optical  glass  the  striae  are  often  of  microscopic  size,  but 
can  be  detected  by  placing  the  glass  across  the  path  of  a  beam 
of  parallel  rays,  and  viewing  it  through  a  lens  (or  combina- 
tion of  lenses  such  as  an  eye-piece),  when  any  striae  present 
appear  as  either  shadows  or  brighter  lines.  In  the  absence 
of  sunlight,  a  small  light  a  few  yards  away  will  answer 
the  purpose. 

Glass  is  often  subjected  to  internal  strain,  due  to  tension 
or  compression,  or  both.  In  the  case  of  ordinary  glass,  this 
is  of  little  or  no  consequence  unless  the  strain  leads  to  rupture, 
but  in  optical  glass  strain  appreciably  changes  the  optical 


GLASS  AND  ENAMELS  319 

properties,  and  as  the  strains  are  seldom  distributed  sym- 
metrically in  a  lens,  the  resulting  image  is  distorted.  It 
fortunately  happens  that  this  defect  (unlike  striae,  colour, 
etc.),  can  be  eliminated  by  very  careful  annealing  of  the 
finished  glass,  without  re-melting  it. 

Hardness  in  optical  glasses  is  necessary  to  enable  it  to 
resist  abrasion  while  it  is  being  made  into  lenses,  and  to 
enable  the  lenses  to  resist  accidental  abrasion  or  scratching. 
Durability  implies  the  power  to  resist  the  chemical  action  of 
air,  water,  or  other  things  which  may  touch  it ;  glasses  show 
wide  differences  with  respect  to  this  property.  For  want  of 
sufficient  durability,  certain  glasses  which  would  otherwise 
prove  very  valuable  for  optical  purposes,  are  quite  useless — 
as  borate  flint  glasses  and  phosphate  crown  glasses.  On  the 
other  hand,  the  best  glasses — as  certain  hard  crown  glasses 
and  borosilicate  crown  glasses — are  not  appreciably  affected 
by  atmospheric  moisture  and  carbon  dioxide,  or  by  the  organic 
matter  of  dust  which  might  be  left  on  glass  surfaces  touched 
by  the  fingers.  Other  glasses  (chiefly  very  dense  flint 
glasses  high  in  lead) ,  though  resistant  to  moisture  and  carbon 
dioxide,  are  liable  to  become  spotted  by  the  action  of  the 
organic  matter  of  dust.  The  presence  of  much  alkali,  and 
of  barium  oxide  and  boric  oxide,  tends  to  make  glass  suscep- 
tible to  chemical  action  of  the  air.  Modern  chemical  glass- 
ware, such  as  the  well-known  Duroglass,  for  example,  is 
very  resistant  even  to  the  prolonged  action  of  hot  water, 
of  alkalies,  and  of  acids. 

Milkiness  is  occasionally  found  in  optical  glass,  and  has 
been  traced  by  Professor  Turner  and  his  colleagues  at 
Sheffield,  and  by  Fenner  and  Ferguson  in  the  United  States 
(/.  American  Ceramic  Soc.,  i,  468,  1918)  to  the  action 
of  very  small  amounts  of  sulphates  and  chlorides — chiefly 
the  former — the  direct  cause  of  the  milkiness  being  attributed 
to  separation  of  clouds  of  minute  crystals  of  cristobalite. 

With  reference  to  refraction,  the  refractive  property  of  a 
glass  is  usually  expressed  by  the  refractive  index,  as  measured 
for  sodium  light  (yellow)  represented  by  the  D  line  of  the 
spectrum.  For  this  refractive  index  the  symbol  n»  is  used. 


320  SILICA   AND   THE  SILICATES 

The  refractive  indices  for  other  lines  are  similarly  represented 
by  »A,  n0,  nv,  etc. 

The  dispersive  power  (or  dispersion)  is  the  power  of  a 
glass  to  spread  or  disperse  white  light  into  a  spectrum  of 
different  colours.  The  mean  dispersion  is  defined  as  the 
width  of  the  spectrum  between  the  lines  C  and  F,  and  is 
measured  by  the  difference  between  the  refractive  indices 
for  C  and  F — that  is,  mean  dispersion  (A)  —nv—nQ.  Another 
important  quantity,  known  as  v  (it  has  not  received  any 
special  name),  is  obtained  by  dividing  (the  refractive  index 
less  one)  by  the  mean  dispersion,  that  is,  v=(n-D—i)/(nJ?—n0). 

The  optical  defects  of  simple  lenses  give  rise  to  chromatic 
aberration  and  other  irregularities,  but  these  can  be  remedied 
by  suitable  conbinations  of  lenses  made  of  different  kinds 
of  glass. 

In  an  article  on  the  "Manufacture  of  Optical  Glass  "  (/. 
American  Ceramic  Soc.,  2,  422,  1919),  W.  S.  Williams  and 
C.  C.  Rand  state  that  with  a  glass  consisting  of  44*4  per  cent. 
of  SiO2,  46-0  of  PbO,  4-1  of  K2O,  3-5  of  Na2O,  2'O  of  CaO, 
and  0*4  of  As2O3,  the  refractive  index  and  v  varied  as 
follows  in  different  melts  : — 

Refractive 

index      1-6282    1-6283    1-6268      1*6278    1*6288    1-6248    1-6242    1-6221    1-6285    1.6265 
v  36-7        36-7        36-7          36-8        36-5        36-6       37-2        37'4        36'6        36-? 

The  effects  of  changes  in  composition  on  the  average 
values  of  the  index  and  of  v  are  shown  in  the  following  : — 


No.  of 

Refractive 
average 

Glass.           melts. 

Si02. 

K20. 

Na20. 

,  CaO.  ZnO.  B2O3 

.  PbO. 

BaO.  As208. 

index. 

v. 

Medium  flint 

10 

44'4 

5'° 

3*5 

3-0     — 

—  . 

44-0 

— 

0-2 

1-62446 

37-2 

IO 

4'1 

3  '5 

3-0     — 

— 

44-0 

— 

O'2 

1-62741 

36-9 

\\ 

10 

44*4 

3'5 

2-0     

— 

46-0 

— 

0'4 

1-62658 

36-8 

Light  barium 

crown     .  . 

7 

48-1 

7'5 

i-o 

lO'I 

4'5 

— 

28-3 

0'4 

1-56907 

57'3 

99                ft 

7 

48-0 

6-1 

2*0 

—     IO'O 

4-0 

— 

29*5 

1*4 

i'57343 

567 

7 

47-6 

6-0 

2'O 

-      9-8 

4-0 

— 

30-5 

i*5 

1-57700 

56-5 

99                99 

5 

46*0 

6-1 

2-0 

—    10-5 

5*5 

— 

29*5 

i  "5 

1-57990 

567 

The  effects  are  due  partly  to  differences  in  raw  materials, 
and  partly  to  volatilization  of  materials  in  melting  the  glass. 

Attention  may  be  directed  to  Dr.  W.  Rosenhain's  "  Cantor 
Lectures  on  Optical  Glass  "  (lyondon,  1916),  and  to  several 


GLASS  AND  ENAMELS  321 

other  interesting  papers  on  optical  glass  in  the  Journal  of  the 
American  Ceramic  Society  by  C.  N.  Fenner,  G.  W.  Morey, 
H.  S.  Roberts,  and  R.  J.  Montgomery  (2,  102, 146,  543,  1919, 
and  j,  404,  1920,  respectively).  By  plotting  out  the  known 
optical  glasses  according  to  their  refractive  indices  and 
dispersion  constants,  Montgomery  finds  there  are  twenty- 
three  ordinary  types  of  optical  glass,  each  type  comprising 
a  number  of  glasses  quite  similar  in  optical  properties, 
composition,  and  method  of  manufacture.  They  range 
from  borosilicate  crown  glasses  through  various  ordinary 
crowns  and  barium  crowns,  barium  flints,  and  borosilicate 
flints,  and  through  ordinary  flints,  to  the  densest  silica 
flint  glasses. 

In  the  same  journal  (/,  730,  1918)  T.  D.  Yensen,  in 
a  paper  on  "  Magnesia  Ware,"  incidentally  points  out  that 
large  crystals  separate  from  electrically -fused  magnesia,  and 
that  these  crystals  are  cubic  and  isotropic,  and  combine 
very  high  refractive  index  with  low  dispersion — a  valuable 
combination  of  properties  in  material  for  lenses. 

A  special  variety  of  optical  glass  is  made  into  spectacles 
for  use  by  glass-furnace  operators  to  avoid  injury  to  their 
eyes.  This  glass  contains  cerium,  derived  from  waste 
material  left  after  extraction  of  the  relatively  small  proportion 
of  thorium  oxide  from  monazite.  (See  report  on  monazite 
in  "The  Mineral  Industry  of  the  British  Empire  and  Foreign 
Countries,"  1920.) 

STRASS  AND  IMITATION  GEMS. 

The  glass  or  "  paste  "  used  for  making  imitation  jewels 
is  made  from  the  purest  obtainable  materials,  including  the 
colouring  oxides.  The  base  of  all  these  products  is  the 
paste  or  strass,  generally  made  of  white  sand  or  quartz, 
red  or  white  lead,  and  potassium  carbonate,  with  boric  acid 
or  borax.  After  thoroughly  melting  the  product  is  quenched 
in  water,  the  melting  and  quenching  being  repeated  several 
times.  For  the  finest  strass,  the  potash  is  used  in  the  form 
of  purified  caustic  potash.  The  paste  is  thus  a  heavy  lead 
C.  21 


322  SILICA   AND  THE  SILICATES 

glass  or  crystal,  which  is  strongly  refractive  and  very  brilliant. 
The  powdered  strass  is  mixed  with  the  appropriate  colouring 
materials,  subjected  to  prolonged  fusion,  and  cooled  so  slowly 
as  to  be  annealed  in  the  melting  pot.  It  is  finally  broken 
up,  and  the  pieces  of  glass  are  used  for  making  imitation  gems. 
Varying  proportions  of  the  materials  are  given,  but  in  all 
cases  lead  oxide  forms  approximately  half  the  whole  weight, 
and  silica  about  one-third  the  whole  weight.  The  boric  acid, 
or  borax,  is  usually  about  2  per  cent,  of  the  total  quantity. 

The  colouring  material  added  to  1000  parts  by  weight 
of  strass  or  paste  is  : 

For  imitation  topaz,  40  parts  glass  of  antimony,  i  part 
purple  of  Cassius  ;  alternatively  10  parts  iron  oxide 
may  be  used. 

For  imitation  ruby,  glass  of  antimony  as  for  topaz,  but 
with  a  little  more  purple  of  Cassius,  and  with  longer 
heating. 

For  imitation  sapphire,  15  parts  cobalt  oxide. 
For  imitation  emerald,  8  parts  copper  oxide,  and  0'2  part 

chromium  oxide. 

According  to  W.  Stein  ("Die  Glasfabrikation,"  1862), 
for  coloured  imitation  gems  may  be  used  not  only  every  lead 
crystal  or  flint  glass,  but  also  mixtures  containing  little  lead 
and  even  leadless  mixtures. 


ENAMELS. 

Enamels  are  vitreous  or  glassy  materials  which,  when 
melted,  can  adhere  firmly  to  the  surface  of  metal  or  of 
pottery.  Those  on  pottery,  as  mentioned  in  the  preceding 
section,  are  merely  glazes  of  a  particular  type.  These  latter 
have  not  as  a  rule  been  vitrified  before  being  applied  on  the 
ware,  but  enamels  for  metals  are  commonly,  though  not 
invariably,  melted  before  they  are  applied  to  the  metal  ; 
in  most  enamelled  iron  advertisement  tablets,  the  enamel 
is  applied  in  the  raw  state,  as  on  pottery. 

Enamelling  on  metal  was  practised  in  early  times,  the 
enamel  being  for  a  very  long  period  sunk  into  cells  (or 


GLASS  AND  ENAMELS  323 

cloisons).  The  base  of  the  enamel  is  generally  a  colourless 
flux  or  glass — consisting  of  silica,  red  lead,  and  potash— in 
which  are  suspended  particles  of  opaque  material,  generally 
tin  oxide.  Above  900°  C.  tin  oxide  may  have  undergone 
combination  or  solution,  resulting  in  diminished  opacity. 
Tin  oxide  has  been  substituted  by  titanium  oxide  or  antimony 
compounds,  and  opacity  may  also  be  obtained  by  using 
arsenious  oxide,  calcium  phosphate,  cryolite  (natural  or 
artificial),  or  fluorspar.  Coloured  enamels  are  best  prepared 
by  adding  the  colouring  material  to  a  flux  previously  pre- 
pared by  the  usual  method  of  fusing  the  components  together. 
Enamels  are  hard  or  soft  according  as  they  require  a  very 
high  or  a  lower  temperature  to  melt  them ;  the  harder 
enamels  contain  relatively  more  silica. 

The  softer  enamels  are  more  readily  affected  by  weather 
influences.  Great  fusibility  may  be  secured,  if  necessary, 
by  mixing  borates  with  the  silicates.  Pure  metals  and  hard 
enamels  make  the  best  combination  for  durability.  Enamels 
containing  much  soda  or  potash  are  more  liable  to  crack 
and  to  separate  from  the  metal  than  enamels  with  more 
red  lead  in  them.  All  transparent  enamels  can  be  made 
opaque  by  adding  a  calx  consisting  of  lead  oxide  (litharge), 
and  tin  oxide,  made  by  calcining  a  mixture  of  lead  and  tin. 
The  enamel  is  very  finely  powdered  and  well  washed  with 
water,  and  the  metal  on  which  it  is  to  be  applied  is  carefully 
cleaned  by  immersion  in  dilute  acid — nitric  acid  for  copper, 
sulphuric  acid  for  silver,  and  hydrochloric  acid  for  gold ; 
the  acid  is  thoroughly  cleaned  off,  and  finally  the  powdered 
enamel  is  applied  evenly,  and  after  drying,  the  metal  object 
is  transferred  to  the  muffle,  where  it  is  heated  to  bright  pale 
redness.  In  contrast  with  pottery  or  glass,  the  firing  of 
enamel  only  takes  a  few  minutes,  and  annealing  is  hardly 
ever  required. 

J.  Minneman  (Trans.  American  Ceramic  Soc.,  13, 
514,  1911)  gives  as  the  best  soft  enamel  flux  found  in  the 
course  of  his  investigation  a  high  lead  flint  glass  of  the 
formula  0-5  PbO,  0-5  K2O,  1*5  SiO2,  made  from  12-9  parts 
white  lead,  10  parts  nitre,  and  9  parts  silica.  This  flux 


324  SILICA   AND  THE  SILICATES 

compared  favourably  in  every  way  with  a  German  com- 
mercial soft  flux.  For  a  hard  enamel  he  found  the  following 
very  suitable  :  0-5  PbO,  0-5  K2O,  2*0  SiO2,  made  from 
12  *9  parts  white  lead,  10  parts  nitre,  and  12  parts  silica. 

Champleve  Enamelling  has  the  design  outlined  by 
cutting  hollows  in  the  plate  with  the  graver,  leaving  raised 
lines  and  bands  of  metal  between ;  the  hollows  are  filled 
with  powdered  enamel,  which  is  melted,  and  the  whole  is 
then  ground  smooth. 

Cloisonne  Enamelling  is  another  mode  of  inlaid  enamel- 
ling, in  which  thin  metal  strips,  fixed  to  the  metal  plate  by 
older  or  by  the  enamel  itself,  form  a  raised  outline,  and  the 
cells    enclosed  are  filled  with  enamel    and    treated  as  in 
champleve. 

The  basse-taille  process  is  also  a  combination  of  metal- 
work  in  the  form  of  engraving,  carving,  and  enamelling.  A 
design  was  first  engraved,  followed  by  carving  into  a  bas- 
relief  (low  relief,  below  the  general  surface  of  the  metal), 
so  that,  when  the  enamel  was  melted  on,  the  design  showed 
through  the  translucent  enamel,  the  varying  thickness  of 
which  produced  differences  of  tint. 

Plique-a-jour  is  a  kind  of  cloisonne  work  in  which, 
by  removing  the  ground  upon  which  they  are  fired,  the 
pattern  is  left  outlined  by  strips  of  metal,  the  spaces  en- 
closed by  the  metal  strips  being  filled  with  translucent 
enamel. 

Painted  enamels  are  usually  done  on  copper,  but  some- 
times on  silver  or  gold,  the  plate  of  metal  being  slightly 
convex.  The  metal  was  cleaned  with  dilute  sulphuric  acid, 
then  coated  evenly  over  front  and  back  surfaces  with  white 
enamel  powder,  and  afterwards  fired.  As  a  result  the  metal 
ground  was  concealed.  The  design  is  next  laid  on  in  a 
"  flux,"  or  more  fusible  and  more  transparent  enamel,  which 
may  be  either  white  or  some  colour  which  can  take  other 
colours  later.  Painted  enamels  were  introduced  in  the 
fifteenth  century  in  I/imoges. 

In  Grisaille  Painted  Enamels  the  white  enamel  is 
painted  thickly  in  the  lighter  parts  and  thinly  in  the  grey 


GLASS  AND  ENAMELS  325 

parts,  giving  great  light  and  shade  with  a  slight  sense  of 
relief. 

In  Coloured  Painted  Enamels  the  painting  was  executed 
on  the  smooth  fluxed  surface  in  transparent  enamel  colours, 
or  powdered  glasses  mixed  with  a  suitable  vehicle,  each 
colour  being  vitrified,  by  a  separate  firing  in  a  muffle.  Touches 
of  gold  in  lines,  etc.,  are  sometimes  added  to  improve  the 
effect,  and  foil  is  occasionally  used  under  the  transparent 
enamel. 

Two  new  enamelling  processes  were  brought  out  by 
A.  Fisher  not  many  years  ago.  The  first  is  an  inlay  of 
transparent  enamel  resembling  plique-ti-jour  without 
separation  of  the  colours  by  cloisons,  for  if  (as  in  painted 
enamels  and  basse-taille  enamels)  the  melted  enamels  do 
not  run  together,  there  should  be  no  need  for  the  separation 
in  this  process,  the  result  being  a  clear  transparent  subject 
in  colour.  The  second  new  process  consists  of  a  coloured 
enamel  relief,  the  colour  of  the  enamel  permeating  the  whole 
depth  of  the  relief ;  it  resembles  the  relief  on  della  Robbia 
ware,  but  the  colour  is  only  on  the  surface  of  the  latter,  and 
another  difference  is  that  one  has  a  fresco  surface  whilst  the 
surface  of  the  other  is  highly  glazed.  The  shapes  of  the 
various  parts  of  the  relief  are  selected  for  the  different 
enamels,  and  these  enamels  are  melted  together,  in  the 
mould  of  the  relief,  which  is  finished  with  lapidary's  tools. 

Miniature  Enamel  Painting  is  not  true  enamelling ; 
the  white  enamel  is  fired  upon  the  plate  as  in  other  cases, 
but  the  colours  used  are  not  vitreous  compounds — that  is, 
not  true  enamels — but  are  raw  oxides,  etc.,  with  a  little 
flux  added  (not  combined).  These  colours,  after  being 
painted  on  the  white  enamel,  are  made  to  adhere  to  the 
surface  by  partially  fusing  the  enamel. 

Enamelled  Sheet  Iron  has  been  largely  used  for  street 
names,  advertisements  such  as  railway  notices,  etc.  Soft 
sheet  steel  is  much  used  for  domestic  hollow-ware  Cast 
iron  is  also  enamelled,  but  is  used  mainly  for  sanitary  goods, 
such  as  sinks,  lavatories,  tubs,  etc. 

The  metal,  after  being  thoroughly  cleaned  by  pickling 


326  SILICA   AND  THE  SILICATES 

in  an  acid  bath,  is  covered  with  a  thin  coating  of  ground 
enamel,  which  need  not  be  opaque,  and  generally  contains 
a  little  cobalt  or  nickel  oxide.  When  this  coating  is  dry, 
the  article  is  fired  at  a  temperature  a  little  under  1000°  C. 
in  a  muffle  furnace.  The  ware  is  next  scrubbed  to  remove 
scale,  and  a  thicker  coating  of  opaque  white  cover  enamel 
is  applied  over  the  ground  enamel ;  the  articles  are  again 
fired,  but  at  a  somewhat  lower  temperature.  Sometimes  a 
third  coat  is  applied,  and  sometimes  a  single  white  coating 
is  put  directly  on  the  metal,  the  coating  of  ground  enamel 
being  omitted.  The  surface  may  finally  be  decorated  by 
painting  or  transfer  printing  in  enamel  colours,  or  by  photo- 
graphs. 

The  enamels  used  for  cast-iron  goods  are  soft  borosilicate 
frits,  made  opaque  by  tin  oxide.  If  the  enamel  is  for  a 
ground  or  slush  coat,  it  is  made  more  infusible  by  adding 
clay,  felspar,  flint,  etc.,  to  the  fritted  material  sent  to  the 
mill  to  be  ground  dry.  If  it  is  a  cover  enamel  it  may  have 
additions,  usually  supplementary  opacifying  agents ;  the 
finely  ground  material  is  ready  for  enamelling.  Ground 
material  for  a  slush  coat  is  mixed  with  water  to  the  thick- 
ness of  paint.  The  slush  coat  serves  to  protect  the  casting 
from  oxidation  during  the  first  heating,  and  to  form  a  close 
union  with  the  iron  (which  the  cover  enamel  could  not  do). 
The  cover  enamel  should  fit  the  iron  exactly,  both  as  regards 
melting  point  and  coefficient  of  expansion.  A  slight  differ- 
ence in  the  coefficient  will  cause  the  enamel  to  craze  (if  too 
soft),  or  to  peel  or  chip  off  (if  too  hard).  Most  of  the  good 
ground  coats  for  cast-iron  enamels  contain  borax,  red  lead, 
and  sodium  nitrate  (J.  H.  Coe,  Trans.  American  Ceramic  Soc., 
13,  531,  1911)  ;  this  author  states  that  the  use  of  cobalt 
oxide  in  these  is  of  doubtful  value.  Other  writers  take  the 
view  that  the  cobalt  oxide  improves  the  enamel.  F.  H. 
Riddle  (Trans.  American  Ceramic  Soc.,  p,  646,  1907)  gives 
the  range  of  favourable  composition  of  cover  enamels  for 
cast  iron  as  :  alkalies,  0*2  to  0*6  equivalent,  preferably  near 
the  lower  limit  (on  account  of  solubility),  and  mostly  as 
felspar,  but  some  as  nitrate,  with  B2O3  as  borax  ;  fluoride, 


GLASS  AND  ENAMELS  327 

not  less  than  0-083,  but  satisfactory  at  0-125  (perhaps  up 
to  0*25  or  0*3  if  introduced  in  the  form  of  barium  fluoride 
or  cryolite)  ;  barium,  0*0  to  0*45  equivalent ;  the  ratio  of 
BaO+PbO  to  the  remaining  RO  bases  should  be  about 
10  :  5  to  7  ;  boric  acid,  0*05  to  0-3  equivalent,  preferably  as 
borax  when  permissible  ;  lead  oxide,  0*1  to  0*4  equivalent 
(in  high  proportions  it  produces  transparency  and  yellow 
colour)  ;  zinc  oxide,  O'O  to  O'i  equivalent,  aids  in  opacifying, 
and  tends  to  overcome  the  yellow  cast  from  lead,  but  is 
rather  infusible ;  alumina,  O'l  to  0*25  equivalent  (the 
higher  the  alumina  the  less  tin  is  required)  ;  silica,  0-75  to 
1-25  equivalent,  should  be  kept  as  high  as  possible,  as  these 
glasses  are  generally  basic  ;  tin  oxide,  0*2  to  0*25  equivalent 
is  mostly  used,  but  0-15  equivalent  (about  7-5  per  cent.) 
works  very  well ;  the  most  likely  formula  would  be  about 
as  follows  :  0-40  PbO+BaO,  0*15  K2O,  0*15  CaO,  0*05  ZnO, 
0-25  Na2O,  0-15  A12O3,  ro  SiO2,  0*2  B2O3,  0'2  SnO2— the 
lower  the  lead  and  the  higher  the  barium  the  better. 

H.  Vogel  (Keramische  Rundschau,  23,  109,  1915)  states 
that  enamel  adheres  on  hard  white  cast  iron  better  than  on 
the  softer  grey,  so  that  it  is  advantageous  to  obtain  a  thin 
layer  of  white  iron  on  the  inner  surface  of  a  casting  to  lay 
the  enamel  on.  This  is  attained  by  keeping  the  moulding 
sand  for  the  inside  (that  is,  for  the  core)  as  cool  as  possible, 
and  wetter  than  the  sand  for  the  outside.  Also,  by  melting 
iron  containing  sulphur,  a  hard  iron  resembling  the  white 
is  produced ;  this  is  effected  by  dusting  the  surface  of  the 
core  with  sulphur  powder.  Dusting  of  the  mould  with 
graphite  is  to  be  avoided,  as  some  graphite  would  adhere  to 
the  iron  even  after  careful  cleaning. 

H.  F.  Staley  (/.  American  Ceramic  Soc.,  i,  98,  1918) 
states  that,  in  ground  coat  enamels  for  cast  iron,  the  potash 
felspar,  flint,  and  clay,  could  be  substituted  for  one  another 
without  changing  the  fusibility  of  the  enamel,  the  ratios 
being  100  felspar  :  66|  flint  :  40  clay,  and  that  the  "  flint 
equivalent  "  (obtained  by  thus  substituting  all  the  clay  and 
felspar  of  a  ground  coat  formula  by  flint,  and  expressing  the 
total  flint  calculated  as  the  number  of  pounds  to  1000  pounds 


328  SILICA   AND   THE  SILICATES 

of  melted  batch)  gives  a  fair  indication  of  the  refractori- 
ness. 

With  regard  to  Sheet-iron  Enamels,  R.  R.  Danielson 
(Trans.  American  Ceramic  Soc.,  18,  343,  1916)  points  out 
that  in  order  to  make  a  good  enamel,  as  the  ground  coat 
must  soften  sufficiently  for  the  more  fusible  cover  enamel  to 
fuse  into  the  ground,  part  of  the  flint  might  simply  be  added 
to  the  frit  for  the  ground  coat,  thus  producing  a  tougher 
ground  coat.  All  combinations  of  cobalt  and  nickel  oxides 
were  found  to  give  good  tough  enamels ;  probably  0*4  per 
cent,  cobalt  oxide  with  075  per  cent,  nickel  oxide  would 
give  the  best  results  for  a  ground  coat  of  high  quality. 
White  grounds  are  not  certain  enough  for  commercial 
practice,  and  the  use  of  cobalt  and  nickel  oxides  is  strongly 
recommended  for  the  production  of  good  coats.  J.  B.  Shaw 
(Trans.  American  Ceramic  Soc.,  n,  103,  1909)  gives  the 
following  limits  of  variation  in  enamels  for  sheet  steel : 
ground  coat  formula,  0-15  to  075  K2O,  o  to  0'6  Na2O, 
0-14  to  0*64  CaO,  o  to  0*06  CoO,  .  .  .  MnO2,  O'i  to  0-5 
A12O3,  1*1  to  17  SiO2,  0*2  to  0*5  B2O3 ;  cover  enamel  for- 
mula, o  to  0'6o  K2O,  o  to  0-65  Na2O,  0*2  to  O'6o  CaO,  o  to  0-5 
A12O3,  i-o  to  1-8  SiO2,  0-2  to  more  than  0-42  B2O3.  He 
states  that  for  producing  opacity,  SnO2  may  be  either  partly 
or  entirely  replaced  by  ZnO+Sb2O3,  with  equally  good 
results,  0-13  equivalent  of  ZnO+Sb2O3,  or  of  SnO2,  being 
sufficient  to  produce  a  good  white  opacity  by  using  two 
coats  of  white. 

H.  F.  Staley  (Trans.  American  Ceramic  Soc.,  13,  502, 
1911)  in  a  paper  on  the  "  Control  of  Fusibility  in  Enamels/' 
states  that  the  following  oxides  have,  pound  for  pound,  the 
same  fluxing  power  :  K2O  from  soluble  salts,  Na2O  from 
soluble  salts,  BaO  from  BaCO3,  ZnO,  PbO.  Calling  the 
fluxing  power  of  any  one  of  these  oxides  unity,  0*8  Ib.  of 
either  B2O3  or  BaF2  gives  i  unit  of  fluxing  power,  the 
fluxing  power  factor  of  these  substances  being  i  J.  One  unit 
of  fluxing  power  is  given  by  0*6  Ib.  of  CaF2,  which  thus 
has  a  fluxing  power  factor  of  if.  Calling  the  refractoriness 
of  an  addition  of  I  Ib.  dry  flint  i,  it  takes  T|  Ib.  of  a 


GLASS  AND  ENAMELS 


329 


particular  felspar,  or  of  bone-ash,  to  give  i  unit  of  refrac- 
toriness, so  that  this  felspar  and  bone-ash  have  a  refrac- 
toriness factor  of  f .  The  full  statement  of  the  fusibility  of 
an  enamel  is  embodied  in  the  following  : — 


FLUXES  (A). 
K2O  from  soluble  salts 
Na2O  from  soluble  salts 
BaO  from  BaCO3 
CaO  from  CaCO3 
ZnO 
PbO 


B203 
BaF2 


Xi  = 


CaF 


I 


Na3AlF6  f 


XI|= 


REFRACTORIES  (B). 

Flint  x  i  = 
Felspar  Xy= 
Bone-ash  X§  = 
Sand  Xz= 

(y  and  z  must  be  determined 
experimentally  for  each 
felspar  and  each  sand. 

CaO  from  CaCO3  has  the 
fluxing  value  here  as- 
signed for  small  amounts 
only.) 


The  quotient  obtained  by  dividing  the  sum  of  the  fluxing 
units  (A)  by  the  sum  of  the  refractory  units  (B)  represents 
the  fluidity  ratio  (A/B).  The  ratios  have  been  checked 
repeatedly  in  large  batches  used  in  regular  factory  practice. 
But  though  all  the  enamels  given  in  the  paper  have  the  same 
fusibility,  they  differ  in  other  properties,  such  as  opacity, 
lustre,  heat  range,  liability  to  craze,  etc. 

Reference  may  be  made  to  numerous  other  papers 
relating  to  enamels,  in  the  Transactions  and  Journal  of  the 
American  Ceramic  Society,  by  H.  F.  Staley,  R.  D.  Landrum, 
J.  B.  Shaw,  B.  B.  Poste,  Iv.  J.  Frost,  and  R.  E.  Brown,  more 
especially  8,  172,  1906  ;  13,  489,  1911  ;  14,  489,  516,  740, 
756,  1912  ;  75,  620,  1913  ;  16,  577,  1914 ;  //,  137*  J73> 
I9I5  ;  18,  570,  762,  i9l6  ;  *9>  J46,  339>  I9I7  ;  J->  z>  422, 
502,  534,  640,  703,  1918  ;  /.,  2,  32,  944,  1919.  Also  to 
articles  by  R.  Vondracek  in  Sprechsaal,  41,  475,  493,  507, 
1908,  and  42,  201,  220,  1909. 

J.  Schaefer  (Keramische  Rundschau,  25,  75,  1917)  states 


330  SILICA   AND   THE  SILICATES 

that  5  per  cent,  of  zinc  sulphide  in  enamels  containing  about 
5  per  cent,  cryolite  gave  good  covering  power  at  800°  C., 
and  was  not  decomposed  into  zinc  oxide,  the  latter  having 
little  or  no  covering  power.  The  covering  power  of  the 
sulphide  was  as  great  as  that  of  tin  oxide,  but  the  white 
colour  and  lustre  were  not  so  good.  At  1000°  C.  the  covering 
power  of  the  zinc  sulphide  enamel  nearly  disappeared,  and 
the  enamel  became  yellowish,  both  effects  arising  from 
double  decomposition  between  zinc  sulphide  and  oxides  of 
the  ground  enamel.  Zinc  sulphide  is  not  decomposed  in 
glazes  not  readily  fusible,  but  the  lustre  is  slight.  The  trials 
were  all  made  on  sheet  iron,  but  zinc  sulphide  has  also 
proved  very  suitable  for  cast-iron  enamels.  Special  zinc 
sulphide  is  now  produced  which  gives  bright  enamels.  It 
cannot  be  used  for  coloured  enamels,  owing  to  decomposition 
and  resulting  discoloration,  so  the  only  other  heavy  metals 
which  can  be  present  are  lead,  antimony,  and  arsenic. 

F.  Kraze  in  1914  published  a  paper  on  "  Enamel  Powder 
Technolog)^/'  of  which  an  abstract  appears  in  Trans.  Ceramic 
Soc.,  77,  Abs.  150,  1918.  The  finely  ground  dry  enamel 
is  applied  to  the  red-hot  iron  vessel  either  by  dipping  the 
latter  into  the  powder  and  turning  it  round  or  by  dusting 
the  powder  on  to  it.  In  the  case  of  lead  enamels,  subsequent 
heating  in  the  muffle  is  unnecessary,  but  leadless  powders 
require  this  subsequent  heating.  For  lead  enamel  the 
dipping  process  is  preferable  to  dusting,  from  sanitary 
considerations,  as  well  as  to  avoid  loss  of  enamel.  The 
wet  coating  was  rendered  possible  by  the  introduction  of 
clay,  which  is  added  in  the  mill  to  the  granulated  enamel 
frit ;  it  makes  the  enamel  more  refractory,  but  also  more 
resistant  to  chemical  action. 

The  constitution  of  these  enamels  is  illustrated  by  the 
following  molecular  formulae  : — 

I.  0-20  PbO,  0-12  BaO,  0-20  Na2O,  0-29  ZnO,  0*07  CaO, 
0-12  K2O,  0-12  A12O3,  0-80  SiO2,  0*16  B2O3. 

II.  0-20-0  ZnO,  074-070  Na2O,  0-06-0-11  K2O,  o-O'ig 
CaO,  0-12-0-15  A12O3,  0-0-13  Sb2O3,  0-77-0-64  SiO2,  i -14-0-68 
B2O3,  0-043-0  P2O5,  0-34-0  SnO2,  0-60-0-86  F. 


GLASS  AND  ENAMELS  331 

I.  is  an  actual  lead  enamel  powder  for  dipping. 

II.  gives  the  limiting  values  of  a  series  of  practical 
leadless  powder  enamels  for  dusting.     These  powders  are 
suitable  for  cast  iron,  and  are  not  satisfactory  without  a 
ground.     The  ground  also  was  formerly  powdered,  but  is 
now  laid  on  moist.     The  molecular  proportions  of  formula  II. 
to  the  left  furnish  a  very  easily  fusible  powder,  and  to  the 
right  a  powder  which  will  run  on  steep  surfaces,  but  whose 
viscosity  on  hot  walls  inclined  less  than  45°  is  yet  satisfactory. 

For  powder  enamels  the  raw  materials  should  be  finely 
ground  and  well  mixed  before  fritting.  For  enamels  laid 
on  moist,  the  opacifying  oxides  are  not  put  in  the  frit,  but 
added  at  the  mill,  which  is  not  permissible  for  powder 
enamels,  because  of  the  bad  effects  resulting  from  differences 
in  specific  gravity.  Zirconia  is  troublesome  in  this  way, 
but  a  very  fine  leather-brown  cloudy  effect  has  thus  been 
obtained.  This  enamel  with  earthy  zirconia  proved  extra- 
ordinarily resistant  to  very  sudden  temperature  changes 
and  to  percussion.  Zirconia  enamel  has  good  covering 
power,  and  is  fairly  acid-proof,  and  resistant  to  reducing 
influences.  Powder  enamels  containing  zirconia  can  be 
applied  direct  on  iron  without  using  a  ground  enamel. 
In  powders  rich  in  lead,  zirconia  seems  to  have  a  stronger 
clouding  tendency  even  than  in  leadless  ones. 

Tin  oxide  up  to  10  per  cent.,  fritted  into  the  powder 
enamel,  produced  in  lime-soda  borosilicates  no  satisfactory 
clouding,  and  none  at  all  with  up  to  3  per  cent.  Fluorine 
in  addition  to  tin  oxide  greatly  strengthens  the  cloudiness, 
though  alone  it  has  only  slight  effect.  This  clouding  effect 
was  still  further  strengthened  by  applying  the  powder  on 
iron  at  a  bright-red  heat  and  then  heating  to  smoothness  in 
the  enamel  muffle.  Similarly,  fluorine  causes  cloudiness  of 
otherwise  clear-dissolving  antimony  oxide,  calcium  phos- 
phate, barium  phosphate,  zirconium  phosphate,  and  zirconia. 
Kraze  has  introduced  barium  and  zirconium  phosphates 
into  enamels  by  using  barium  carbonate  and  zirconia  re- 
spectively with  sodium  phosphate.  Such  barium  powder 
enamels  have  brighter  lustre  than  corresponding  powders 


332  SILICA    AND   THE  SILICATES 

with,  calcium  phosphate.  The  clouding  effect  of  zirconia 
is  strengthened  by  using  it  as  zirconium  phosphate,  besides 
the  combination  with  fluorine.  In  enamels  containing 
lime,  cloudiness  is  produced  by  tungstic  acid.  Clouded 
enamels  can  also  be  produced  by  merely  adding  enough 
cryolite,  fluorspar,  or  sodium  silicofluoride.  The  cloudiness 
of  fluoride  powder  enamels  is  ascribed  mainly  to  the  presence 
of  free  silicon  fluoride  besides  aluminium  fluoride,  but  the 
enamel  can  be  acid-proof  and  also  clouded  after  complete 
removal  of  silicon  fluoride,  even  if  the  enamel  contains  no 
other  clouding  agents,  or  is  clear  when  it  contains  no  fluorine. 
The  feebly  opaque  enamel  freed  from  silicon  fluoride  can  be 
made  to  give  a  good  white  covering  even  over  a  not  too  dark 
ground  enamel.  The  ground  enamel  for  cast  iron  needs 
neither  cobalt  nor  nickel.  It  is  impossible  to  get  acid-proof 
fluoride  enamels  by  a  single  melting  in  a  crucible.  The 
melting  should  be  repeated  until  satisfactory  trials  are 
obtained,  the  crucible  being  left  open  for  the  escape  of 
gases  (silicon  fluoride,  etc.). 

BRITISH  DEVELOPMENTS. 

The  colossal  dislocation  of  trade  and  industry  caused  by 
the  great  world  catastrophe  revealed  many  weaknesses, 
and  prominent  among  them  were  the  practical  absence  of 
home  facilities  for  the  production  of  certain  varieties  of  glass, 
notably  resistance  glass  for  scientific  purposes,  and  (though 
not  to  the  same  extent)  optical  glass.  In  pre-war  times  only 
one  British  firm  (Messrs.  Chance,  of  Smethwick)  was  engaged 
in  making  optical  glass,  and  although  this  firm  had  been 
long  established,  the  output  represented  only  a  fraction  of 
the  nation's  requirements  even  under  normal  conditions, 
and  with  the  utmost  extensions  of  the  works  was  woefully 
inadequate  to  meet  the  sudden  emergency.  In  order  to 
meet  the  urgent  demands  which  were  soon  foreshadowed* 
energetic  efforts  were  made  by  a  number  of  scientific  men  to 
provide  a  satisfactory  basis  on  which  the  manufacture  of 
optical  and  resistance  glasses  could  be  undertaken  in  existing 


GLASS  AND  ENAMELS  333 

glassworks  and  in  others  to  be  erected.  Among  those  inti- 
mately concerned  in  this  important  work  were  Sir  Herbert 
Jackson  and  Dr.  T.  R.  Merton  at  King's  College  (London), 
Dr.  W.  Rosenhain  and  his  colleagues  at  the  National  Physical 
Laboratory,  F.  W.  Branson  at  Leeds,  and  Dr.  M.  W.  Travers 
at  Walthamstow,  soon  reinforced  by  Professor  W.  E.  S. 
Turner  and  his  band  of  research  workers  at  the  newly 
established  Glass  Department  at  the  University  of  Sheffield. 
The  results  of  their  combined  labours  is  sufficiently  well 
known,  and  some  interesting  particulars  may  be  found  in  the 
Society  of  Chemical  Industry's  Annual  Reports  on  the 
progress  of  Applied  Chemistry  (which  reports  began  for  1916 
and  part  of  1915),  and  in  the  Journal  of  the  Society  of  Glass 
Technology  (which  society  was  founded  in  November,  1916). 
The  investigations  of  Professor  Turner  and  his  co-workers 
showed  that  some  glasses  of  English  manufacture  were 
superior  to  the  famous  Jena  glass  as  regards  chemical 
resistance,  a  striking  testimony  to  quality. 

The  increasing  use  of  automatic  and  semi-automatic 
machinery  for  bottles  and  all  kinds  of  small  glassware  has 
the  effect  of  augmenting  the  output  while  decreasing  the 
cost  of  production .  Another  imp ortant  indication  of  progress 
is  the  substitution  of  gas-fired  furnaces  for  the  wasteful 
coal-fired  furnaces.  Recuperative  furnaces,  especially  of  the 
Hermansen  and  the  Stein  types,  seem  to  be  attracting  con- 
siderable attention,  no  doubt  in  large  measure  owing  to  the 
lower  cost  of  labour  attending  their  use.  The  question  of 
annealing  has  also  received  much  attention. 

On  the  whole  it  may  fairly  be  claimed  that  remarkable 
progress  has  been  made  in  the  British  glass  industry.  Further 
evidence  of  this  is  afforded  by  the  number  of  different  kinds 
of  glass  now  being  produced,  one  firm  alone  being  engaged 
in  the  production  of  more  than  seventy  new  kinds  of  glass. 
It  is  particularly  noteworthy  in  this  case  that,  instead  of 
being  developed  on  the  basis  of  analyses  of  foreign  glasses — 
as  was  attempted  in  the  first  instance  without  much  satis- 
faction— these  are  the  results  of  a  successful  study  (extending 
over  a  period  of  three  years)  of  the  influence  of  the  several 


334  SILICA   AND   THE  SILICATES 

component  oxides  on  one  another.  For  further  details 
reference  may  be  made  to  C.  J.  Peddle's  series  of  papers  on 
"  The  Development  of  Various  Types  of  Glass  "  in  volume  4 
of  the  Journal  of  the  Society  of  Glass  Technology. 

The  figures  for  exports  and  imports  of  glass  for  the  years 
1913  to  1918  are  suggestive,  but  the  variations  in  price 
should  not  be  overlooked. 

TRADE  STATISTICS. 

The  following  figures  show  the  values  (in  •£)  of  the  total 
exports  and  imports  between  this  country  and  foreign 
countries  and  British  possessions  respectively.  The  exports 
include  plate  glass,  flint  glassware,  glass  bottles,  and 
miscellaneous  goods.  The  imports  include  window  and  sheet 
glass,  plate  glass,  flint  glassware,  glass  bottles,  with  tiifling 
amounts  for  miscellaneous  goods.  Prices  should  be  borne 
in  mind. 

I.  Exports  to  Foreign  Countries. 
II.  Exports  to  British  Possessions. 
III.  Imports  from  Foreign  Countries. 
IV.  Imports  from  British  Possessions. 


1913-      i9T4-      I9I5-     1916.     1917.     1918. 

I.  712,149  586,065  437,975  454,078  506,511  442,924 
II.  1,101,616  964,723  805,823  849,572  679,468  609,371 

III.  3,448,983  2,238,330  1,973,007  2,856,894  620,292  290,372 

IV.  437    J'OQi    5,834   33,453  3,244    48 


LITERATURE. 

Bastow,  H.,  "  American  Glass  Practice."     Pittsburgh,  1920. 

Benrath,  H.  E.,  "  Die  Glasf abdication."     Brunswick,  1875. 

Bontemps,  G.,  "  Guide  du  Verrier."     Paris,  1868. 

Bull,  P.,  "  Die  Emaille-Fabrikation."     Hamburg— Bergedorf,  1895. 

Cunynghame,  Sir  H.  H.,  "  Art  Enamelling  upon  Metals."  2nd  edition. 
London,  1901. 

Dralle,  R.,  "  Die  Glasf abdication."     Munich,  1911. 

Eyer,  P.,  "  Die  Eisenemaillierung."     Leipzig,  1907. 

Fisher,  A.,  "  Art  of  Enamelling  upon  Metal."     London,  1906. 

Flamm,  P.,  "  Le  Verrier  du  XIXe  Sidcle."     Paris,  1863. 

Grunwald,  J.,  "  Theory  and  Practice  of  Enamelling  on  Iron  and  Steel." 
Translated  by  Hodgson.  London,  1909.  "  Raw  Materials  for  the  Enamel 
Industry."  Translated  by  Hodgson.  London,  1914. 


GLASS   AND   ENAMELS  335 

Gessner,  F.  M.,  "  Glass-makers'  Handbook."     Pittsburgh,  1891. 

Heaton,  N.,  "  Glass  "  in  Thorpe's  "  Dictionary  of  Applied  Chemistry." 
London. 

Henrivaux,  "  La  Verrerie  au  XXe  Si£cle."     Paris,  1903. 

Hovestadt,  D.  H.,  "  Jena  Glass."  Translated  by  Everett.  London, 
1907. 

Le  Chatelier,  H.,  "  La  Silice  et  les  Silicates."     Paris,  1914. 

Millenet,  L.  E.,  "  L'Emaillage  sur  Metaux."     Paris,  1917. 

Peligot,  E.,  "  La  Verre."     Paris,  1877. 

Powell,  Chance,  and  Harris,  "  Principles  of  Glass-making."  London, 
1883. 

Randan,  P.,  "  Enamels  and  Enamelling."  Translated  by  Salter. 
2nd  edition.  London,  1912. 

Rosenhain,  W.,  "  Optical  Glass  (Cantor  Lectures)."     London,  1916. 

Schnurpfeil,  H.,  "  Schmelzung  der  Glaser."  Vienna,  1906 ;  "  Two 
Hundred  Recipes  for  Glasses  of  all  kinds."  Zurich,  1918. 

Stein,  W.,  "  Die  Glasfabrikation."     Brunswick,  1862. 

Tscheuschner,  E.,  "  Handbuch  der  Glasfabrikation."  5th  edition. 
Weimar,  1885. 

Zschimmer,  E.,  "  Die  Glasindustrie  in  Jena."     1909. 

Journal  of  the  Society  of  Glass  Technology.     Sheffield,  1917  on. 

Transactions  of  the  American  Ceramic  Society.  Easton,  Pa.,  1899  to 
1917- 

Journal  of  the  American  Ceramic  Society.     Easton,  Pa.,  1918  on. 

Transactions  of  the  Optical  Society.     London. 

Transactions  of  the  Physical  Society.     London. 


SECTION  VI.— MISCELLANEOUS  APPLICA- 
TIONS OP  SILICATES 

IN  this  section  are  brought  together  for  consideration  a  few 
silicious  materials,  and  products  obtained  from  silicious 
materials,  which  do  not  fall  under  any  of  the  previous 
headings.  They  comprise  certain  special  products  derived 
from  clay,  shales,  and  slates,  and  from  felspar  and  slag  ; 
also  some  earthy  and  other  silicate  pigments  (ochres,  siennas, 
umbers,  etc.). 

AI,UM. 

The  name  "  alum  "  is  applied  to  each  member  of  a  group  of 
double  sulphates  which  in  the  crystalline  state  are  repre- 
sented by  the  general  formula  R2SO4,  £203(803)3,  24H2O, 
where  R2SO4  represents  potassium,  sodium,  or  ammonium 
sulphate  (or  sometimes  other  similar  sulphates),  and  R2O3 
represents  a  sesquioxide  such  as  alumina,  ferric  oxide, 
chromic  oxide,  or  the  corresponding  oxide  of  manganese, 
etc.  They  all  dissolve  in  water,  and  crystallize  from  aqueous 
solution,  usually  in  the  form  of  octahedra  or  cubes ;  octa- 
hedra  when  perfect  are  bounded  by  eight  equilateral  triangles 
arranged  in  two  pyramidal  groups  of  four  each  with  the 
bases  in  both  groups  forming  a  square  in  common,  and  each 
group  of  four  meeting  in  a  common  apex.  The  only  alums 
with  which  we  are  concerned  here  are  those  containing 
aluminium  along  with  potassium,  sodium,  or  ammonium, 
known  respectively  as  potassium  alum,  sodium  alum,  and 
ammonium  alum. 

Potassium  Alum,  Potash  Alum,  or  Common  Alum, 
K2SO4,  A12O3(SO3)3,  24H2O,  is  the  substance  to  which  the 

336 


APPLICATIONS  OF  SILICATES  337 

name  "  alum  "  was  first  given,  and  it  is  still  often  termed 
simply  "  alum."  It  occurs  naturally  as  the  mineral  kalinite, 
forming  fibrous  crystals  (occasionally  octahedra),  or  as  an 
efflorescence  on  aluminous  minerals  ;  it  is  found  in  some 
quantity  in  certain  volcanic  districts,  produced  by  the 
action  of  volcanic  gases  upon  felspathic  rocks  and  minerals. 
A  more  abundant  mineral  is  alunite,  or  alumstone,  in  which 
the  aluminium  sulphate  is  basic,  and  which  has  the  com- 
position indicated  by  the  formula  K2SO4,  A12O3(SO3)3, 
2A12(OH)6.  Alurjite  occurs  in  France,  Italy,  Hungary, 
Greece,  Asia  Minor,  China,  and  Australia.  In  China  a 
mountain  nearly  1900  feet  in  height  is  officially  reported  to 
consist  of  alunite. 

In  the  production  of  alum  from  alunite,  the  old  process 
is  still  used  in  Italy,  at  I,a  Tolfa  (near  Civita  Vecchia),  the 
broken  mineral  being  first  carefully  calcined  at  a  low  red 
heat,  in  kilns  or  simply  in  heaps.  About  a  third  of  the 
weight  is  lost  (chiefly  water),  and  alum  and  insoluble  alumina 
remain.  The  roasted  material  is  left  exposed  to  the  air  for 
several  months,  being  moistened  occasionally,  and  the  sludge 
thus  obtained  is  treated  with  water  at  70°  C.,  and  the  clear 
liquid  decanted  off  and  concentrated.  Alum  crystals  are 
deposited  on  cooling  ;  they  are  tinged  reddish  by  ferric 
oxide,  which  may  be  got  rid  of  by  recrystallization.  The 
product  is  known  as  Roman  alum,  and  is  distinguished  for 
its  purity. 

In  the  modern  process,  which  is  used  in  other  parts  of 
Europe,  the  calcination  is  carried  out  at  a  higher  temperature, 
and  sulphuric  acid  is  added  to  the  product,  aluminium 
sulphate  being  formed  along  with  the  less  soluble  alum. 
The  alum  is  crystallized  out,  and  the  aluminium  sulphate  is 
then  obtained  by  evaporating  the  mother  liquor,  or  alterna- 
tively sufficient  potassium  sulphate  is  added  to  form  alum 
with  all  the  aluminium  sulphate. 

Most  of  the  alum  manufactured  in  England  was  formerly 
made  from  alum  shale,  alum  schist,  and  the  like,  which  occur 
abundantly  at  Whitby,  and  at  Hurlet  and  Campsie  (near 
Paisley),  as  well  as  in  numerous  localities  on  the  Continent. 

C.  22 


338  SILICA    AND   TEE  SILICATES 

These  minerals  consist  of  aluminium  silicate,  finely  divided 
iron  pyrites,  and  bituminous  substance.  The  coarsely 
broken  alum  shale,  with  alternating  coal  layers,  is  built  up 
into  heaps,  which  are  set  on  fire.  As  the  roasting  proceeds, 
fresh  quantities  of  shale  are  added,  the  temperature  being 
carefully  regulated — by  sprinkling  water  over  the  surface 
from  time  to  time — so  as  to  avoid  loss  of  sulphur  dioxide, 
and  formation  of  slag.  Aluminium  sulphate  is  produced, 
and  by  long  exposure  to  the  air  more  oxygen  is  absorbed, 
ferrous  sulphate  and  ferric  oxide  being  formed.  The  roasted 
material  is  treated  with  the  mother  liquor  from  the  alum, 
and  afterwards  with  water — in  each  case  leaving  it  in 
contact  overnight — the  liquors  being  run  into  settling  tanks, 
and  allowed  to  deposit  calcium  sulphate,  ferric  oxide,  and 
other  suspended  impurities,  and  then  taken  away  to  be 
concentrated.  When  the  shale  contains  little  or  no  magnesia 
(as  at  Hurlet  and  Campsie)  surface  evaporation  in  a  re- 
verberatory  furnace  is  resorted  to,  followed  by  further 
concentration  in  leaden  pans,  and  the  liquid  is  afterwards 
mixed  with  the  proper  amount  of  dry  potassium  chloride, 
which  soon  dissolves  on  agitation.  The  liquid  portion  is 
run  off  about  five  days  later,  and  the  large  crystals  left  are 
washed  and  finally  recrystallized.  When  (as  at  Whitby) 
the  shales  contain  magnesia  in  fair  quantity,  surface  evapora- 
tion would  soon  form  a  crust  of  magnesium  sulphate,  which 
would  greatly  hinder  further  evaporation.  The  evaporation 
therefore  takes  place  in  leaden  vessels  straight  away,  and 
when,  after  several  stages,  the  specific  gravity  of  the  solution 
reaches  1*4  to  1*5,  it  is  run  into  a  tank  and  mixed  with 
the  calculated  amount  of  potassium  chloride  or  sulphate, 
agitation  being  kept  up  to  cause  a  deposition  of  small 
crystals  termed  "alum  meal/'  When  there  is  enough  iron 
sulphate  present  to  convert  potassium  chloride  into  sulphate, 
the  more  soluble  chloride  is  generally  used  instead  of  the 
sulphate,  as  the  resulting  iron  chlorides  (both  ferric  and 
ferrous)  are  very  soluble,  and  the  formation  of  iron  alum  is 
prevented.  Kxcess  of  chloride  would  result  in  the  formation 
of  the  very  soluble  aluminium  chloride,  which  would  be  lost. 


APPLICATIONS  OF  SILICATES  339 

The  brownish  alum  meal  is  drained  and  washed  twice 
with  cold  water,  which  removes  the  ferruginous  mother 
liquor,  leaving  the  crystals  nearly  pure.  The  meal  so  purified 
is  dissolved  in  as  little  water  as  possible,  and  left  for  about 
eight  days  before  draining  off  the  mother  liquor,  which  is 
used  to  dissolve  more  meal.  The  mother  liquor  gives  more 
alum  when  partly  evaporated,  and  afterwards  gives  ferrous 
sulphate. 

In  1845  Spence  first  used  ammonium  sulphate  instead 
of  potassium  sulphate,  thus  producing  ammonium  alum. 
About  the  same  time  he  produced  alum  from  the  bituminous 
shales  below  the  South  I,ancashire  coal  seams.  The  shale 
was  slowly  roasted  in  heaps,  using  a  little  fuel  only  to 
start  combustion,  and  the  soft,  porous,  light-red  product 
was  digested  at  110°  C.  for  about  48  hours  in  lead-lined 
pans  with  sulphuric  acid  of  specific  gravity  1*35.  The 
subsequent  addition  of  ammonia  (or  ammonium  sulphate) 
results  in  the  formation  of  ammonium  alum.  The  solution 
is  kept  agitated,  and  the  small  crystals  thus  produced  are 
drained,  washed  with  mother  liquor  from  "  block  alum," 
and  dissolved  in  a  minimum  amount  of  steam  under  pressure. 
A  little  size  is  added  to  the  solution  to  promote  precipitation 
of  insoluble  matter,  and  the  clear  liquid  is  run  ofl  to  form 
"  block  alum  "  after  standing  ;  this  is  drained,  the  mother 
liquor  forming  about  one-fourth  of  the  total  weight.  A 
ton  of  ammonium  alum  is  produced  by  this  method  from 
about  15  cwt.  of  shale.  Spence's  process  enabled  the  alum 
to  be  ready  for  sale  in  a  month,  whereas  by  the  old  process 
it  took  a  year  to  convert  the  crude  shale  into  alum  for  the 
market. 

Alum  is  produced  by  adding  potassium  sulphate  to 
aluminium  sulphate  prepared  in  any  convenient  way.  Very 
pure  alum  is  obtained  from  the  aluminium  sulphate  produced 
from  cryolite,  the  yield  of  alum  being  three  times  the  weight 
of  the  cryolite. 

The  preparation  of  alum  from  ordinary  felspar  (which 
contains  potassium  and  aluminium  in  the  proper  proportions, 
but  in  the  form  of  silicates)  has  often  been  proposed,  but 


340  SILICA   AND   THE  SILICATES 

commercially  it  is  difficult  owing  to  the  high  temperature 
necessary  to  decompose  the  silicates. 

At  present,  most  of  the  alum  of  English  manufacture  is 
made  either  from  shale  or  from  the  aluminium  sulphate 
produced  from  China  clay  or  bauxite. 

Potassium  alum  usually  crystallizes  in  colourless  octa- 
hedra,  but  the  crystals  formed  at  ordinary  temperatures 
in  the  presence  of  basic  alum  are  cubes  often  with  dull 
surfaces  due  to  the  basic  salt ;  Roman  alum  generally  forms 
cubes  for  the  same  reason.  At  40°  C.,  even  when  basic 
salts  are  present,  octahedra  are  produced.  The  solubility 
of  alum  (potassium  or  ammonium)  increases  greatly  as  the 
temperature  rises. 

Potassium  alum  is  strongly  acid  in  reaction,  and  has  a 
rather  sweet  astringent  taste.  The  aqueous  solution  when 
heated  is  decomposed,  depositing  a  basic  alum,  especially  from 
a  dilute  solution.  Advantage  is  often  taken  of  this  to  add  a 
little  alum  (not  enough  to  give  the  characteristic  taste)  to 
impure  water,  when  the  gelatinous  precipitate  takes  with  it 
most  of  the  organic  impurities,  including  any  colouring 
matter  which  may  be  present. 

Alum  is  nearly  insoluble  in  a  saturated  solution  of 
aluminium  sulphate.  Exposure  to  air  causes  alum  crystals 
to  assume  a  white  appearance  on  the  surface,  due  to  absorp- 
tion of  ammonia  from  the  air,  a  basic  salt  being  formed.  The 
crystals  lose  no  water  below  30°  C.,  but  lose  all  of  it  at  100°  C., 
though  slowly. 

Alum  melts  in  its  water  of  crystallization  at  92-5°  C., 
and  when  heated  to  redness  it  is  converted  into  "  burnt 
alum,"  a  porous,  friable  material,  which  dissolves  in  watei 
slowly.  When  heated  to  whiteness,  alum  leaves  only  alumina 
and  potassium  sulphate. 

"  Burnt  alum,"  heated  to  redness  with  one-third  of  its 
weight  of  carbon,  gives  Homberg's  phosphorus,  which  is 
spontaneously  inflammable  owing  to  the  presence  of  finely- 
divided  potassium  sulphide. 

Neutral  Alum,  along  with  sodium  sulphate,  is  formed 
by  adding  caustic  soda  or  sodium  carbonate  to  alum  solution 


APPLICATIONS  OF  SILICATES  341 

until  the  precipitate  first  produced  is  only  just  redissolved 
on  shaking  (two-thirds  of  the  acid  being  then  neutralized). 
This  solution  of  a  neutral  basic  alum  is  used  as  a  mordant 
by  dyers,  because  it  easily  gives  up  the  excess  of  alumina 
to  the  fabric.  Alum  is  much  used  as  a  mordant  in  the 
dyeing  industries,  and  for  the  production  of  other  aluminium 
mordants,  such  as  the  acetate,  etc.,  used  in  dyeing  and 
printing,  and  also  for  shower-proofing  fabrics.  Alum  is 
used  for  sizing  paper,  and  for  making  fire-proofing  materials. 
Alum  used  for  the  more  delicate  dyes,  as  alizarin  red,  must 
be  extremely  pure,  as  very  slight  amounts  of  iron  cause  dull 
tints  to  be  produced. 

Aluminium  sulphate  now  economically  replaces  alum 
in  most  of  its  uses. 

Sodium  Alum  or  Soda  Alum,  Na2SO4,  Al2SOs(SOs)s, 
24H2O,  occurs  in  South  America  and  J  apan  as  mendozite.  It 
was  first  prepared  by  Zellner  in  1816,  from  a  solution  con- 
taining the  two  sulphates.  Ostwald  questioned  its  existence, 
which  has  since  been  established  by  Wadmore,  who  obtained 
octahedral  crystals  of  the  composition  indicated.  The 
crystals  do  not  appreciably  effloresce  in  the  air,  though  the 
contrary  statement  has  often  been  made. 

Sodium  alum  may  be  prepared  in  quantity  by  heating 
and  keeping  at  50°  to  60°  C.  a  solution  containing  667  grams 
of  crystallized  aluminium  sulphate  per  litre,  and  adding  a 
solution  containing  142  grams  of  anhydrous  sodium  sulphate 
per  litre,  until  the  specific  gravity  of  the  liquid  becomes 
1*35.  On  cooling — preferably  at  a  temperature  of  10°  to 
25°  C.— sodium  alum  crystallizes  out;  at  28°  C.  crystal- 
lization is  very  slow,  and  below  10°  C.  sodium  sulphate  is 
deposited. 

In  the  process  patented  by  F.  M.  Spence  and  D.  D.  Spence 
(Bng.  pat.  5644,  March  26,  1900),  a  saturated  solution  of 
sodium  sulphate  along  with  crystals  of  the  same  substances 
are  run  together  into  a  solution  of  aluminium  sulphate  in 
which  solid  aluminium  sulphate  is  suspended.  A  mass  of 
well-defined  sodium  alum  crystals  is  obtained.  Alterna- 
tively, solid  aluminium  sulphate  may  be  added  to  the  mixed 


342  SILICA    AND   THE  SILICATES 

solution  and  crystals  of  sodium  sulphate,  or  to  a  similar 
mixture  of  sodium  chloride  solution  and  crystals. 

Sodium  alum  crystallizes  in  regular  octahedra.  At 
ordinary  temperatures  it  is  much  more  soluble  (in  water) 
than  potassium  or  ammonium  alum,  and  accordingly  it  is 
more  difficult  to  purify  from  iron.  If  it  were  possible  to 
purify  it  easily  by  crystallization,  it  would  be  largely  used 
instead  of  other  alums,  owing  to  the  comparative  cheapness 
of  sodium  compounds. 

Ammonium  Alum,  (NH4)2SO4,  A12O3(SO3),  24H2O, 
occurs  in  Bohemia,  and  in  the  crater  of  Mount  Etna,  as  the 
mineral  tschermigite.  Its  preparation  is  similar  to  that  of 
potassium  alum,  a  solution  of  aluminium  sulphate,  prepared 
in  any  convenient  way,  being  mixed  with  the  proper  amount 
of  ammonium  sulphate,  and  the  resulting  alum  separated 
and  purified  by  crystallization. 

lyike  potassium  alum  and  sodium  alum,  ammonium  alum 
crystallizes  in  regular  octahedra,  with  24  molecules  of  water. 
At  ordinary  temperatures  it  is  somewhat  less  soluble  in 
water  than  potassium  alum.  When  heated  the  crystals  lose 
water  and  sulphuric  acid,  swelling  up  and  forming  a  porous 
mass ;  at  a  sufficiently  high  temperature  only  alumina 
remains,  and  this  is  a  useful  method  for  preparing  very  pure 
alumina. 

The  general  properties  and  uses  of  ammonium  alum  are 
practically  the  same  as  those  of  potassium  alum,  except  as 
regards  the  action  of  strong  heat. 

Aluminium  Sulphate,  A12O3(SO3)3,  i8H2O,  or  A12(SO4)3, 
i8H2O.  This  forms  the  chief  constituent  of  the  mineral 
known  as  alunogen,  feather  alum,  hair  salt,  or  halotrichite, 
which  occurs  in  volcanic  districts,  and  also  in  pyritous  shales. 
It  is  abundant  in  the  coal  measure  shales  of  Hurlet  and 
Campsie,  near  Paisley.  It  is  found  in  various  continental 
localities  and  also  in  the  United  States  and  South  America. 
It  often  contains  considerable  proportions  of  ferrous  oxide, 
with  small  amounts  of  magnesia,  potash,  etc.  Aluminite 
or  websterite  is  a  hydrated  basic  aluminium  sulphate,  the 
composition  of  which  is  represented  by  the  formula  A12O3, 


APPLICATIONS  OF  SILICATES  343 

SO3,  9H2O.  It  is  found  at  Newhaven  and  other  places  on 
the  Sussex  coast.  On  the  Continent  it  occurs  in  several 
localities  in  France  and  Germany.  Alunite,  alumstone,  or 
alum  rock  consists  of  the  basic  sulphate  just  referred  to — 
but  with  less  water — associated  with  potassium  sulphate, 
of  the  composition  K2SO4,  3A12(SO3),  6H2O.  The  mineral 
occurs  in  abundance  at  I^a  Tolfa  near  Civita  Vecchia  (Italy), 
and  is  also  found  in  Greece,  Hungary,  France,  etc.  It  is 
the  product  of  the  action  of  sulphur  gases  upon  trachytic 
rocks  containing  much  felspar. 

Aluminium  sulphate  is  produced  by  the  action  of  sul- 
phuric acid  on  either  hydrated  aluminium  oxide  or  aluminium 
silicate.  The  most  important  raw  mateiials  now  available 
for  the  manufacture  are  China  clay  and  bauxite.  China 
clay  (kaolin),  as  stated  in  Section  II.,  is  a  very  pure  clay, 
rep  resented  approximately  by  the  formula  A12O3, 2SiO2, 2H2O. 
In  this  country  it  is  only  found  in  workable  quantities  in 
Devonshire  and  Cornwall,  but  it  also  occurs  in  France  and 
other  continental  countries,  as  well  as  in  North  America. 
Bauxite  is  an  impure  aluminium  hydroxide  with  varying 
amounts  of  ferric  oxide  and  silica.  It  is  found  in  the  north- 
east of  Ireland,  in  the  south  of  France,  and  in  other  countries. 

Aluminium  Sulphate  from  China  Clay. — This  method 
of  manufacture  is  now  practised  extensively,  the  process 
being  based  on  that  described  in  Pochin's  patents  (No.  231, 
January  30, 1855,  and  No.  562,  March  6, 1856) .  Cornish  China 
clay  is  mostly  used,  as  free  as  possible  from  iron  oxide  and  grit. 
The  clay,  ground  very  finely  in  a  mill,  is  sieved,  dried,  and 
then  calcined  at  a  dull-red  heat,  the  temperature  being  raised 
gradually.  The  clay  loses  20  to  25  per  cent,  of  its  weight, 
owing  to  expulsion  of  the  moisture  (10  to  15  per  cent.), 
and  most  of  the  water  of  hydration  (previously  combined 
with  the  alumina  and  silica).  The  calcined  clay  still  has 
about  3  per  cent,  of  water,  and  is  introduced  into  a  lead- 
lined  wooden  vat  charged  with  the  proper  amount  of  sul- 
phuric acid  at  85°,  the  specific  gravity  at  this  temperature 
being  96°  Tw.  The  materials  at  once  react  strongly,  and 
after  15  minutes  with  constant  stirring,  the  contents  of 


344  SILICA   AND   THE  SILICATES 

the  vat  are  run  into  lead-lined  wooden  wagons  in  which  the 
reaction  keeps  on  for  some  time  and  the  pasty  material 
gradually  becomes  solid.  The  solid  block  is  finally  by 
mechanical  means  cut  and  crushed  to  produce  a  coarse 
powder.  The  product,  sold  under  the  name  of  "  alum 
cake/'  contains  all  the  silica,  iron,  and  other  impurities. 
About  60  per  cent,  of  the  alumina  of  the  China  clay  is  con- 
verted into  aluminium  sulphate. 

The  commercial  product  termed  "  white  sulphate  of 
alumina  "  is  obtained  from  alum  cake  by  lixiviating  the 
coarse  powder  with  water  (or  with  weak  liquors  from  earlier 
extractions)  in  steam-heated  vats  lined  with  lead ;  the 
clear  liquid  (after  settling)  is  decanted,  by  means  of  a  hinged 
pipe,  into  lead-lined  evaporators,  heated  by  steam  coils, 
and  is  concentrated  to  112°  Tw.  at  the  boiling  point  of  the 
liquid  (about  115°  C.).  The  syrupy  liquid  is  next  run  into 
shallow  tiled  troughs,  and  solidifies  there  on  cooling,  but  by 
inserting  in  the  troughs,  before  solidification,  leaden  partitions, 
the  product  is  obtained  as  uniform,  rectangular  blocks. 
The  "  white  sulphate  of  alumina  "  thus  prepared  usually 
contains  about  14  per  cent,  alumina,  and  0*25  per  cent, 
ferric  oxide,  with  practically  no  insoluble  matter  ;  a  special 
grade  of  the  material  contains  17*5  per  cent,  alumina. 

Aluminium  Sulphate  from  Bauxite.— As  compared 
with  China  clay,  bauxite  is  more  readily  soluble  in  acid,  and 
needs  no  preliminary  calcination,  but  it  contains  a  rather 
large  amount  of  iron  oxide.  The  use  of  bauxite  instead  of 
China  clay  was  proposed  by  L,e  Chatelier  in  1858. 

According  to  P.  and  F.  M.  Spence's  patent  of  1875 
(No.  1704,  May  7),  the  bauxite  is  first  digested  with  dilute 
sulphuric  acid,  aided  by  steam,  until  the  acid  is  neutralized, 
and  after  letting  the  insoluble  matter  settle,  the  solution 
is  evaporated  to  100°  Tw.,  and  run  off  into  lead  coolers 
with  shallow  partitions.  The  yellowish-green  solidified 
product  forms  blocks  18  or  20  ins.  square,  each  weighing 
about  i  cwt.  It  contains  a  little  iron  and  free  acid,  and 
is  used  in  making  brown  and  other  papers  (but  not  the 
finest),  for  precipitating  sewage  and  refuse  liquids,  and 


APPLICATIONS  OF  SILICATES  345 

for  clarifying  and  decolorizing  water  on  a  large  scale. 
Its  general  composition  is  about  14  per  cent,  alumina  (or 
47  per  cent,  aluminium  sulphate),  0*28  per  cent.  Fe2O3, 
0*32  per  cent.  FeO,  about  35  per  cent,  combined  SO3,  and 
0*45  per  cent,  free  SO3,  with  0*06  per  cent,  insoluble  matter, 
the  remainder  being  water.  Instead  of  producing  a  solid, 
the  mixed  sulphates  may  be  used  in  a  liquid  state. 

The  commercial  aluminium  sulphate  which  is  sold  as 
"  concentrated  alum  "  and  "  alferite  "  is  a  modern  product 
of  similar  composition  made  by  charging  coarsely-powdered 
bauxite  (usually  a  mixture  of  Irish  and  French  bauxites)  into 
a  lead-lined  vat  containing  boiling  dilute  sulphuric  acid  (at 
about  112°  C.)  of  96°  Tw.  After  6  hours'  boiling,  weak 
liquors  are  added  to  reduce  the  specific  gravity  to  70°  Tw. 
(measured  at  the  boiling  point).  After  settling,  the  clear 
liquor  is  decanted,  and  evaporated  in  lead-lined  vessels 
until  the  specific  gravity  reaches  112°  Tw.  (boiling).  It  is 
then  run  into  partitioned  coolers,  to  solidify  into  blocks  or 
slabs,  averaging  13*8  per  cent,  alumina,  0*7  ferric  oxide,  and 
O'l  per  cent,  insoluble  matter.  French  bauxite  alone  would 
give  great  difficulties  in  the  clarification,  but  with  mixed 
Irish  and  French  bauxites  a  clear  liquid  is  easily  obtained. 
The  solution  should  retain  a  little  free  acid,  as  the  fully 
neutralized  liquid  is  very  slow  in  settling. 

Aluminium  sulphate,  as  prepared  by  any  ordinary 
method,  always  contains  appreciable  quantities  of  iron,  and 
it  is  doubtful  whether  any  of  the  numerous  purification 
processes  is  really  satisfactory  on  a  commercial  scale. 

"  Pure  aluminium  sulphate  "  is  prepared  from  pure 
alumina,  which  is  obtained  from  bauxite  by  the  "  alkali 
fusion "  process.  The  finely-powdered  bauxite  is  mixed 
with  soda-ash  to  give  the  proportion  1*0  A12O3  :  1*0  to  1*2 
Na2CO3,  and  the  mixture  is  strongly  heated  in  a  reverberatory 
furnace  for  5  hours,  with  frequent  stirring.  Carbon 
dioxide  is  driven  off,  and  the  sodium  oxide  (soda)  combines 
with  alumina  and  ferric  oxide  to  form  sodium  aluminate  and 
sodium  ferrite.  The  material  is  lixiviated  by  treatment 
with  weak  liquor  from  previous  batches,  and  finally  with 


346  SILICA   AND   THE  SILICATES 

pure  water  ;  the  sodium  aluminate  dissolves  unchanged,  but 
the  sodium  ferrite  is  decomposed  to  form  caustic  soda, 
which  dissolves,  and  ferric  oxide,  which  remains  undissolved. 
The  clear  liquid  is  run  into  a  boiler,  and  saturated 
with  carbon  dioxide  (produced  by  burning  coke  or  by 
decomposing  limestone),  the  liquid  being  heated  to  70°, 
and  well  agitated  all  the  time.  When  all  the  alumina  has 
been  precipitated,  it  is  allowed  to  settle,  the  clear  liquid 
being  decanted  off  and  concentrated  to  facilitate  the  recovery 
of  the  sodium  carbonate  ;  the  alumina  meanwhile  is  drained 
in  a  hydro-extractor. 

Bayer  devised  a  cheaper  method,  for  obtaining  alumina 
from  sodium  aluminate.  It  depends  on  the  fact  that  the 
addition  of  aluminium  hydroxide  with  prolonged  agitation 
effects  the  decomposition  of  the  aluminate,  and  precipitates 
about  70  per  cent,  of  the  alumina  (as  hydroxide),  the  liquid 
now  containing  alumina  and  soda  in  the  proportion  iAl2O3  : 
6Na2O.  The  precipitated  aluminium  hydroxide  (or  hydrate) 
is  allowed  to  settle,  and  the  liquid  is  run  off  into  weak- 
liquor  tanks.  The  precipitate  is  filter-pressed,  enough  being 
left  in  the  vat  to  precipitate  the  next  charge,  and  the  weak 
liquor  may  be  concentrated  for  use  in  reacting  upon  a  fresh 
charge  of  bauxite.  The  aluminium  hydroxide  (or  hydrate 
of  alumina)  so  obtained  should  contain  less  than  i  per  cent. 
of  mineral  impurity,  the  more  objectionable  elements  being 
sodium  and  silicon. 

Alumina  prepared  by  either  process,  when  treated  with 
sulphuric  acid,  gives  very  pure  aluminium  sulphate.  Two 
grades  of  this  sulphate  are  commonly  produced  for  the 
English  market,  one  in  slabs  or  blocks  (containing  14*0  per 
cent,  alumina  and  0*0025  Per  cent,  ferric  oxide),  the  other 
as  powder  (containing  18*0  per  cent,  alumina  and  0*0040  per 
cent,  ferric  oxide). 

Aluminium  sulphate  crystallizes  with  difficulty  in  thin 
six-sided  plates.  The  solubility  in  water  of  both  the 
crystals  and  the  anhydrous  sulphate  (but  especially  the 
former)  increases  with  rise  of  temperature. 

When  heated,  aluminium  sulphate  melts  in  its  water  of 


APPLICATIONS   OF  SILICATES  347 

crystallization,  gradually  forming  a  white  porous  mass  of 
anhydrous  sulphate,  and  at  a  red  heat  pure  alumina  remains. 
The  general  industrial  uses  of  aluminium  sulphate  are 
the  same  as  those  of  ordinary  alum.  It  is  largely  used  in 
paper-making,  and  in  the  preparation  of  red  liquor  as  a 
mordant.  The  coarser  preparations  are  used  for  the  pre- 
cipitation of  sewage,  etc.,  as  previously  mentioned. 


FERTILIZERS. 

Basic  Slag  was  first  produced  by  the  process  patented 
by  S.  G.  Thomas  (1877  to  1879,  beginning  with  No.  4422, 
November  23,  1877).  Melted  pig  iron,  mixed  with  lime  in  a 
lime-lined  Bessemer  converter,  loses  the  bulk  of  its  impurities 
through  oxidation  when  the  blast  is  applied.  The  phos- 
phorus pentoxide  produced  combines  with  lime  to  form 
calcium  phosphate.  The  resulting  basic  slag  contains 
40  to  50  per  cent,  lime,  varying  amounts  of  magnesia,  alumina, 
iron  oxide,  manganese  oxide,  silica,  and  phosphorus  corre- 
sponding with  20  to  50  per  cent,  tricalcium  phosphate. 
Hundreds  of  thousands  of  tons  of  such  slags  have  been 
produced  annually  for  years  past.  It  was  at  first  thought 
that  the  slag  was  of  no  value,  but  it  is  now  known  that  the 
phosphorus  in  the  slag  is  nearly  all  combined  with  calcium 
(as  phosphate),  and  that  the  calcium  phosphate  is  readily 
disintegrated  and  rendered  soluble  in  the  soil,  and  that 
the  ferrous  oxide  in  the  slag  causes  no  injurious  effects. 
A  small  proportion  of  the  phosphorus  (about  1*5  per  cent, 
of  the  whole  amount)  occurs  in  the  slag  as  iron  phosphide, 
which  in  the  soil  becomes  converted  into  phosphate.  The 
phosphate  in  slag  is  not  soluble  in  water.  Attempts  were 
made  to  dissolve  the  phosphates  in  slags,  and  then  to  pre- 
cipitate them  again  for  use  in  manuring,  but  after  some 
time  this  was  found  to  be  unnecessary. 

The  Germans  first  realized  the  importance  of  slag 
fertilizers,  and  actually  purchased  much  of  the  English 
output  before  English  agriculturists  began  to  appreciate  its 


348  SILICA   AND   THE  SILICATES 

value.  To  be  effective  in  soil,  the  slag  must  be  very  finely 
ground. 

Basic  slag  is  particularly  valuable  on  moorland  soils 
containing  much  organic  matter,  and  on  clay  soils  with 
insufficient  lime  on  which  it  is  undesirable  to  use  super- 
phosphate continuously.  It  has  proved  very  valuable  on 
pasture  land. 

The  silica  present  in  combination  in  the  slags,  though 
silica  is  a  normal  constituent  of  certain  plants,  is  not  to  be 
regarded  as  among  the  essential  plant  foods. 

Various  mineral  phosphates,  known  as  coprolites,  phos- 
phorites, etc.,  are  also  used  as  fertilizers.  They  usually 
contain  more  or  less  silicious  matter,  but  the  latter  ranks 
as  an  accidental  impurity,  and  probably  has  little  influence. 

SOILS. 

The  natural  soil  consists  of  the  superficial  disintegrated 
portions  of  the  solid  earth,  mixed  with  more  or  less  material 
derived  from  animal  and  vegetable  sources.  The  com- 
position and  properties  of  a  particular  soil  will  therefore 
depend  mainly  on  the  characters  of  the  rocks  and  minerals 
which  constitute  the  ground  in  the  locality. 

British  soils  may  be  broadly  distinguished  as  sedentary 
soils,  which  have  been  gradually  accumulated  on  the  spot 
through  the  weathering  of  rock  similar  to  that  underlying 
the  soil,  and  drift  soils  (soils  of  transport  or  alluvial  soils), 
which  have  been  conveyed  to  their  present  positions  by  the 
agency  of  running  water  or  ice.  On  steep  slopes  are  mixed 
soils  which  have  largely  been  washed  down  or  rolled  down 
from  a  higher  level,  and  contain  angular  fragments  of 
different  materials ;  such  mixed  soils  are  sometimes  called 
colluvial  soils. 

Farmers  and  others  generally  classify  soils  according  to 
the  ease  or  difficulty  of  working  them,  as  sand,  loam,  clay, 
chalk,  sandy  loam,  etc.  Unfortunately,  these  terms  have 
very  different  meanings  with  different  amounts  of  rainfall, 
and  definite  values  can  only  be  given  through  correlation 
with  the  mechanical  analysis  of  the  soil. 


APPLICATIONS  OF  SILICATES  349 

The  texture  of  soil,  and  its  behaviour  under  cultivation, 
are  determined  by  the  relative  proportions  of  sand,  clay, 
calcium  carbonate,  and  humus  (organic  matter)  in  the  soil. 
Sand  includes  the  coarser  particles, — say  between  0*4  mm. 
and  i  mm.  in  diameter, — which  mostly  consist  of  silica. 
Such  coarse  material  takes  up  very  little  water,  and  the  dry 
grains  have  no  cohesive  power.  Clay  includes  the  finest 
particles  in  the  soil ;  it  will  take  a  large  amount  of  water, 
it  is  plastic  and  impermeable  to  water  when  wet,  it  shrinks 
and  cracks  on  drying,  and  swells  again  on  wetting.  When 
distributed  through  water,  clay  particles  can  be  flocculated 
or  coagulated  by  small  quantities  of  certain  soluble  salts. 
Particles  not  larger  than  0*002  mm.  in  diameter  are  regarded 
as  clay,  and  particles  intermediate  in  size  between  sand  and 
clay  are  classed  as  silt.  Clay  consists  mainly  of  hydrated 
compounds  of  silica  and  alumina  (kaolinite),  but  also 
contains  zeolites  (double  silicates  of  alumina  with  soda, 
potash,  lime,  and  magnesia),  and  also  ferric  hydroxides 
and  very  finely-divided  silica. 

All  fertile  soils  contain  some  calcium  carbonate  finely 
disseminated,  ranging  from  60  per  cent,  or  more  down  to 
mere  traces.  When  the  soil  consists  chiefly  of  calcium 
carbonate  and  clay  it  is  called  a  marl,  but  this  term  unfortu- 
nately is  in  some  districts  applied  to  local  carboniferous 
fireclays. 

The  organic  matter  of  soils  consists  for  the  most  part  of 
vegetable  remains,  and  is  a  variable  mixture  of  complex 
substances,  some  of  which  contain  nitrogen.  The  organic 
matter  of  fertile  soils  largely  consists  of  calcium  humate, 
a  term  which  it  is  sometimes  convenient  to  use,  though  the 
so-called  "  humic  acid  "  (from  which  it  is  derived)  has  not 
a  definite  composition.  Peaty  or  boggy  soils,  in  which 
the  organic  matter  predominates,  are  always  very  dark 
coloured  ;  they  hold  water  strongly,  and  are  very  friable 
when  dry. 

In  the  mechanical  analysis  of  soils,  the  particles  are  graded 
according  to  the  velocity  of  a  current  of  water  that  can 
carry  them,  or  according  to  the  time  required  for  their 


350  SILICA   AND   THE  SILICATES 

deposition  from  suspension  in  water  (the  column  of  water 
being  of  specified  height).  The  grades  recognized  in  the 
United  Kingdom  are  as  under  : 

Stones  and  gravel  . .  3-0    mm.  and  larger. 

Fine  gravel  ..  3-0      ,,  to  i'O    mm. 

Coarse  sand  . .  1*0      ,,  to  0*2       ,, 

Fine  sand    . .          . .  0*2      ,,  to  0*04     ,, 

Silt 0-04    ,,  to  o'oi 

Fine  silt       . .          . .  0*01    ,,  to  0*002   ,, 

Clay  . .          . .  0'002  ,,  and  smaller. 

These  differences  in  the  sizes  of  particles  have  an  important 
bearing  on  the  value  of  soils  for  agriculture.  The  action 
of  silica  and  silicates  of  the  soil  is  very  small  from  a  chemical 
standpoint,  with  possible  exceptions  in  the  case  of  the  zeolites 
and  some  of  the  very  finely-divided  silica.  The  grain  size 
necessarily  affects  the  distribution  of  air  and  water  in  the 
soil,  and  therefore  has  an  important  influence  on  the  life  and 
growth  of  plants.  The  various  bacteria  associated  with 
soils  are  also  powerful  factors  in  connection  with  the  fertility. 

This  very  brief  sketch  may  serve  to  indicate  some  of 
the  very  interesting  problems  arising  in  the  cultivation  of 
the  soil.  It  would  lead  too  far  to  attempt  to  pursue  any 
of  the  lines  of  inquiry  which  readily  suggest  themselves. 

EARTHY  AND  OTHER  SIUCATE  PIGMENTS. 

Pigments  are  insoluble  finely-divided  powders — white, 
coloured,  or  black — which  yield  paints  when  intimately 
mixed  with  suitable  media  (oils,  water,  etc.).  -  Dyes  or  stains, 
which  are  soluble  in  the  vehicles  used,  do  not  fall  within 
this  definition.  Besides  being  insoluble  in  the  media, 
pigments  should  be  chemically  indifferent  to  them.  They 
should  also  be  inert,  i.e.  have  no  action  upon  one  another 
when  mixed  together.  Other  qualities  usually  desirable 
in  pigments  are  durability  or  permanency  ;  body  or  opacity, 
i.e.  the  property  of  completely  covering  and  concealing  the 
underlying  surface  ;  covering  power  or  spreading  power, 


APPLICATIONS  OF  SILICATES  351 

the  extent  to  which  the  pigment  can  be  spread  over  a  large 
surface  ;  tint  and  shade,  and  ease  of  manipulation  with 
vehicles  on  the  palette  are  also  important  in  connection  with 
the  use  of  pigments.  In  oil  paints,  some  pigments  have  the 
useful  property  of  promoting  the  drying  of  the  oil,  white 
lead  being  a  good  example  of  a  pigment  with  drying  quality  ; 
this  drying  quality  depends  on  the  power  of  the  pigment  to 
oxidize  or  promote  the  oxidation  of  the  oily  medium.  As 
to  durability  or  permanency,  an  ideal  pigment  should  be 
"  fast  "  to  light,  unaffected  by  air,  moisture,  acids,  alkalies, 
or  sulphur  compounds,  but  comparatively  few  pigments 
fulfil  all  these  requirements. 

OCHRES,  SIENNAS,  UMBERS,  ETC. 

Yellow  Ochre,  Roman  Ochre,  Roman  Yellow, 
Mineral  Yellow,  Stone  Yellow,  Oxford  Ochre,  Brown 
Ochre,  Golden  Ochre,  Chinese  Yellow,  etc.,  are  argil- 
laceous and  silicious  earths,  often  containing  compounds 
of  calcium,  barium,  etc.,  owing  their  yellow  or  brown  colour 
to  the  presence  of  hydrated  ferric  oxide,  in  varying  amounts 
and  different  degrees  of  hydration.  Apart  from  the  differ- 
ences just  mentioned,  the  tints  of  yellow  ochres  depend 
chiefly  upon  the  amount  of  light-coloured  material  (silica  or 
white  clay,  etc.)  and  the  presence  of  ferric  oxide  (which 
gives  a  ruddier  tint).  When  burnt  (calcined)  all  of  them 
lose  their  essential  water,  and  become  converted  into  red 
ochre,  light  red,  etc.  Ochres  containing  large  amounts  of 
silica  and  clay  (which  often  form  two-thirds  of  the  weight) 
yield  the  less  translucent  and  paler  burnt-red  ochres.  Some 
English  ochres  (from  Oxfordshire,  Derbyshire,  etc.)  are  of 
fine  quality. 

Yellow  ochres  are  generally  prepared  as  pigments  by 
careful  selection,  followed  by  elutriation  or  washing  over, 
soluble  substances  and  coarse  material  being  thus  removed. 
Before  grinding  in  oil,  it  should  be  dried  at  a  little  below 
100°  C.  These  pigments  are  very  durable,  are  inert  with 
other  pigments,  are  not  materially  affected  by  light  or  by 


352  SILICA   AND   THE  SILICATES 

impure  air  or  other  gases,  and  they  can  be  used  with  any 
medium.  Yellow  ochre  is  rarely  adulterated,  because  of  its 
cheapness,  but  the  richer-coloured  varieties  have  some- 
times had  their  colour  heightened  by  turmeric  or  other 
organic  yellow  colour. 

Mars  yellow,  Mars  orange,  Mars  red,  Mars  brown,  Mars 
violet,  are  artificial  ferruginous  pigments  or  artificial  ochres, 
made  by  precipitating  salts  of  iron  (with  or  without  alum 
added)  by  means  of  milk  of  lime,  caustic  soda,  or  caustic 
potash,  drying  the  precipitate,  and  (except  for  yellow) 
calcining  at  a  temperature  depending  on  the  colour  required. 

Brown  Ochre  is  hydrated  ferric  oxide  (limonite)  with 
little  impurity.  An  artificial  brown  ochre  is  prepared  by 
heating  yellow  ochre  to  a  low  red  heat  with  4  per  cent,  of 
common  salt. 

Raw  Sienna  or  Italian  Earth  is  very  similar  to  the  last 
named,  being  a  browner  coloured  yellow  ochre  containing 
a  little  manganese  oxide.  It  is  found  near  Rome  and  in 
Cyprus,  etc.,  and  is  prepared  in  the  same  way  as  yellow 
ochre.  It  is  inert  with  other  pigments.  Raw  sienna  is 
suitable  for  oil,  water-colour,  tempera,  and  fresco  painting. 

Burnt  Sienna  is  an  orange-brown  or  reddish-brown 
pigment,  made  by  carefully  calcining  raw  sienna.  It  is 
durable,  and  suitable  for  every  kind  of  artistic  work. 

Light  Red  or  Burnt  Ochre  is  (or  ought  to  be)  yellow 
ochre  which  has  been  calcined  or  roasted  on  a  red-hot  iron 
plate  for  about  10  hours,  or  otherwise  deprived  of  its  combined 
water.  The  exact  tint  of  the  rather  pale-brownish  or  orange- 
red  depends  on  the  variety  of  yellow  ochre  used,  but  it  may 
be  regarded  as  a  scarlet  modified  by  a  little  yellow  and  grey. 
It  is  opaque  and  durable,  and  inert  with  other  pigments. 

Red  Ochre,  Scarlet  Ochre,  Red  Chalk,  Ruddle, 
Bole,  Sinopis,  etc.,  are  varieties  of  red  haematite  (ferric 
oxide),  with  clay,  chalk,  silica,  etc.,  as  impurities,  and  all 
are  durable  pigments.  They  are  cherry-red  when  finely 
ground. 

Indian  Red,  Persian  Red,  or  Indian  Red  Ochre,  is 
a  variety  of  red  ochre  or  red  haematite,  containing  about 


APPLICATIONS  OF  SILICATES  353 

95  per  cent,  ferric  oxide,  and  has  a  slightly  purplish  tint. 
Some  Indian  red  is  a  calcined  product  from  ferrous  sulphate 
or  from  yellow  ochre.  Some  so-called  Indian  red  is  a 
haematite  from  the  Forest  of  Dean  (Gloucestershire) .  Genuine 
Indian  red  is  durable  in  all  media,  and  is  inert  with  other 
pigments. 

Venetian  Red,  Rouge,  Crocus,  Colcothar,  is  a 
variety  of  red  haematite  less  purplish  than  Indian  red,  but 
the  name  is  now  applied  to  a  particular  quality  of  ferric 
oxide  produced  by  calcining  green  vitriol  (ferrous  sulphate), 
or  by  calcining  ochres  at  a  low  red  heat  for  8  hours. 
Venetian  red  is  less  brownish  than  light  red.  Both  the 
natural  and  artificial  varieties  of  Venetian  red  are  durable, 
and  inert  with  other  pigments.  The  pigment  must,  however, 
be  quite  free  from  soluble  salts,  and  especially  from  sulphates. 

Terre  Verte,  Green  Earth,  Veronese  Earth,  etc., 
are  names  given  to  an  earthy  material  of  very  variable 
composition,  consisting  of  silicates  of  iron,  potassium, 
magnesium,  etc.  The  colour  ranges  from  olive-green  to 
apple-green.  It  is  prepared  for  use  as  a  pigment  by  grinding 
selected  material  to  a  fine  powder,  washing  it  with  rain 
water,  and  then  drying  it.  Sometimes  the  selected  material 
is  heated,  and  then  quenched  in  very  dilute  hydrochloric 
acid  to  remove  ochre  and  other  impurities,  the  undissolved 
portion  being  then  ground,  washed,  and  dried.  Terre 
verte  is  generally  quite  durable  both  in  water-colour  and  oil 
painting.  It  is  inert  with  other  pigments,  but  has  not  much 
body,  and  the  colour  has  low  intensity.  The  best  terre 
verte  comes  from  Italy  and  Cyprus. 

Raw  Umber,  Levant  Umber,  Turkey  Umber,  etc., 
is  a  brown  ferruginous  and  silicious  earth  found  in  many 
places,  but  especially  good  in  Cyprus.  It  differs  from  yellow 
and  brown  ochres  in  containing  a  large  percentage  of  man- 
ganese oxide  and  a  small  percentage  of  water.  English 
specimens  are  poor  in  iron.  For  use  as  a  pigment,  the 
brown  mineral  is  finely  ground,  washed  with  water,  and 
dried.  Raw  umber  is  durable  with  any  of  the  painting  media, 
and  is  inert  with  other  stable  pigments.  A  ferruginous 
c.  23 


354  SILICA  AND  THE  SILICATES 

brown  coal  has  occasionally  been  substituted  for  raw 
umber. 

Burnt   Umber,    Velvet   Brown,    Chestnut   Brown, 

etc.,  result  from  calcination  of  raw  umber,  and  are  equally 
durable  and  equally  inert  with  other  stable  pigments.  The 
colour  is  richer  and  redder  than  that  of  raw  umber,  owing 
to  the  conversion  of  ferric  hydroxide  into  ferric  oxide,  and 
of  some  black  oxide  of  manganese  into  red  oxide. 

Caledonian  Brown  is  a  natural  earth  which  looks  very 
like  burnt  sienna.  It  contains  a  little  combined  water, 
but  consists  mainly  of  the  brown  hydroxides  and  oxides  of 
iron  and  manganese.  When  calcined  it  becomes  brownish 
black.  Either  raw  or  burnt  Caledonian  brown  is  durable, 
inert  with  other  pigments,  and  well  adapted  for  oil  and 
tempera  painting. 

Cappagh  Brown,  Mineral  Brown,  Euchrome,  is  a 
highly  ferruginous  and  manganiferous  earth,  found  at 
Cappagh  Mine,  in  County  Cork.  It  is  redder  than  raw  umber, 
but  similar  in  composition  and  general  characters,  containing 
ferric  hydroxide  and  oxide  with  oxide  or  hydroxide  of  man- 
ganese (probably  a  part  as  red  oxide),  and  a  little  silica, 
alumina,  lime,  etc.  When  heated  to  100°  C.,  Cappagh 
brown  gives  off  much  water,  and  assumes  a  rich  reddish- 
brown  colour  resembling  that  of  burnt  sienna  and  very  like 
that  of  Caledonian  brown.  It  is  suitable  for  oil  paint  or 
water-colour.  It  should  be  dried  (at  60°  to  80°  C.)  previous  to 
grinding  in  oil.  It  is  durable,  and  inert  with  other  pigments. 

Vandyke  Brown  is  the  name  given  to  three  different 
commercial  brown  pigments.  One  is  made  by  calcining 
certain  very  ferruginous  earths  (brown  ochres)  ;  another  is 
merely  a  dark-brown  variety  of  colcothar ;  the  third  is  a 
brown  ferruginous  earth  rich  in  bituminous  matter.  This 
last  is  the  kind  generally  met  with  in  our  country,  and  though 
it  has  a  rich  colour  it  is  by  no  means  durable.  The  other 
two  varieties  are  much  more  durable,  and  are  inert  with 
other  pigments,  but  their  colours  are  not  nearly  so  rich. 

Imitation  Vandyke  browns  are  made  from  mixtures  of 
ochre,  colcothar,  and  lampblack. 


APPLICATIONS  OF  SILICATES  355 

Cologne  Earth  (or  Cullen  Earth),  Cassel  Brown 
(Cassel  Earth),  Rubens  Brown,  Coal  Brown,  etc.,  are 
mostly  soft,  impure  varieties  of  brown  coal  (lignite).  When 
slightly  calcined,  some  of  the  organic  matter  is  carbonized, 
and  the  material  becomes  darker  and  less  changeable  on 
exposure.  Some  so-called  Cologne  earth  is  merely  bitumi- 
nous Vandyke  brown  which  has  been  gently  roasted. 

Mineral  Black,  Slate  Black,  Oil  Black,  or  Black 
Chalk,  is  a  carbonaceous  shale  of  brownish-black  to  blue- 
black  colour,  which,  after  crushing  and  levigation  or  wet 
grinding,  is  suitable  for  a  pigment.  It  is  durable.  Coal 
blacks  are  similar.  . 

Monox,  a  very  light  powder  of  chestnut-brown  colour, 
may  be  mentioned  here.  Its  value  as  a  pigment  has  already 
been  alluded  to  on  pp.  4,  5. 

Kaolin  or  China  Clay,  which  has  already  been  considered 
in  Section  II.,  is  a  durable  pigment,  and  practically  un- 
changeable. In  oils  it  has  not  good  covering  power,  but 
in  water  and  tempera  painting  it  is  a  very  useful  white 
pigment,  and  may  serve  as  a  filler ;  a  "  Chinese  white  " 
for  water-colour  painting  may  contain  as  much  as  3  parts 
China  clay  to  i  part  zinc  oxide.  China  clay  for  pigments 
is  prepared  by  levigation  and  drying  of  the  natural  clay, 
by  which  means  it  is  separated  from  the  chief  impurities 
(mica  and  quartz).  Besides  its  use  for  white  pigments, 
China  clay  serves  as  a  cheap  carrier  for  lake  pigments. 

Steatite  or  Soapstone,  a  hydrated  silicate  of  magnesia, 
also  referred  to  in  Section  II.,  is  used  to  give  a  glazing  effect 
to  some  special  paints. 

Silica  in  the  form  of  an  impalpable  powder  is  too  trans- 
parent to  be  used  alone  as  a  pigment,  though  it  is  very  inert, 
and  exceptionally  stable.  It  is,  however,  very  useful  for 
mixing  with  all  pigments  except  silicious  ochres,  as  a  re- 
inforcing agent  (or  filler). 

Zirconia  also  makes  a  good  white  paint.     (See  p.  269.) 

Ultramarine,  both  natural  and  artificial,  have  been 
dealt  with  at  some  length  in  Section  II.  They  form  fine 
blue  pigments.  Ultramarine  ash  (bluish  grey)  and  mineral 


356  SILICA   AND  THE  SILICATES 

grey  are  two  inferior  products  obtained  in  the  preparation 
of  the  blue  pigment  from  the  natural  lapis  lazuli.  The  blue 
colour  of  ultramarine  is  nearer  to  the  pure  normal  blue  of 
the  solar  spectrum  than  the  blue  of  any  other  pigment,  and 
contains  very  little  violet ;  most  samples  of  artificial  ultra- 
marine differ  in  this  respect.  Natural  ultramarine  is  inert 
to  other  pigments,  but  all  common  acids  (except  carbonic 
acid),  whether  inorganic  or  organic,  discharge  the  colour, 
as  also  does  a  hot  solution  of  alum  immediately  ;  even  cold 
concentrated  solution  of  alum  eventually  bleaches  it,  and 
quickly  bleaches  all  but  the  most  stable  varieties  of  artificial 
ultramarine.  The  texture  of  ultramarine  is  rather  harsh 
and  granular,  but  less  so  when  a  little  Chinese  white  is  mixed 
with  it. 

Other  names  used  for  artificial  ultramarine  are  new  blue, 
French  blue,  permanent  blue,  Gmelin's  blue,  Guimet's  blue. 

Green  ultramarine,  an  intermediate  product  in  the 
manufacture  of  artificial  ultramarine,  is  the  colouring 
matter  of  green  water-paints.  The  more  violet  varieties  of 
artificial  ultramarine — but  not  the  blue  and  greenish- 
blue  pigments — have  their  colour  considerably  weakened 
when  mixed  with  zinc  white,  a  result  not  produced  by  other 
white  substances.  Ultramarine  rich  in  silica — sometimes 
sold  as  "  Oriental  blue  "—resist  the  action  of  alum  best. 
Properly  prepared  artificial  ultramarine  is  permanent  both 
in  oils  and  water.  If  a  thin  wash  on  paper  seems  to  weaken 
as  it  dries,  or  soon  afterwards,  the  change  results  from  the 
action  of  alum  (or  other  aluminium  compound)  in  the  size 
of  the  paper. 

Smalt,  Zaffre,  Royal  Blue,  Saxon  Blue,  or  Dumont's 
Blue,  is  a  very  deep  blue  glass  (almost  black)  containing 
generally  65  to  71  per  cent,  silica,  16  to  21  potash,  6  to  7 
cobalt  oxide,  and  a  little  alumina,  with  oxides  of  iron  and 
nickel  in  inferior  varieties.  It  thus  consists  essentially  of 
cobalt  and  potassium  silicates.  Smalt  is  durable,  and  inert 
with  other  pigments,  but  has  not  good  colouring  or  covering 
power. 

Manganese  Blue  is  a  pigment  said  to  be  obtained  by 


APPLICATIONS  OF  SILICATES  357 

calcining  at  a  red  heat  a  mixture  of  China  clay  or  silica, 
with  manganese  oxide  and  barium  nitrate.  A  similar 
product  is  obtained  by  calcining  a  mixture  of  soda  ash, 
silica,  calcium  carbonate,  and  manganese  oxide. 

Copper  Silicate  has  been  tried  as  a  blue  pigment,  but 
with  little  success. 

Egyptian  Blue. — This  was  a  splendid  blue  colouring 
material,  used  by  the  Romans  for  several  centuries  from  the 
commencement  of  the  Christian  era.  It  occurs  in  several 
beautiful  frescoes  in  the  Vatican,  and  has  been  found  also 
at  Pompeii.  Fouque  found  it  to  be  a  double  silicate  of  calcium 
and  copper,  represented  by  the  formula  CaO,  CuO,  4SiO2, 
without  even  traces  of  alkali.  It  resists  the  action  of  nearly 
all  chemical  reagents,  which  accounts  for  its  perfect  preserva- 
tion in  paintings  1900  years  old.  The  old  method  of  pre- 
paration was  to  mix  well  fine  sand  and  soda  with  copper 
filings,  moisten,  make  into  cakes,  dry,  and  then  fuse  in  an 
earthen  pot  until  the  blue  colour  is  produced — but  the 
temperature  should  not  be  above  a  bright-red  heat.  Fouque 
found  potassium  sulphate  preferable  to  soda  as  a  flux,  and 
he  used  excess  of  bases  (the  Romans  used  excess  of  silica), 
finally  purifying  the  product  by  treatment  with  HC1.  I,. 
Bock  recommends  fusion  of  a  mixture  of  50  parts  quartz, 
21  chalk,  24*4  copper  oxide,  and  4*6  sodium  carbonate,  all 
very  finely  powdered  and  free  from  iron.  The  product,  after 
washing  with  hydrochloric  acid  and  hot  water,  had  the  com- 
position given  above,  and  agreed  closely  in  all  its  properties 
with  the  ancient  colouring  matter.  I^aurie,  Mcl^intock,  and 
Miles  made  a  similar  material.  The  exceptional  qualities 
and  fine  colour  of  Egyptian  blue  should  make  it  well  worth 
manufacturing  again.  (References  :  Compt.  Rend.,  108, 325  ; 
/.  Soc.  Chem.  Ind.,  8,  291  ;  /.  Chem.  Soc.,  no,  ii.,  434,  and 
106,  ii.,  128.) 

LITERATURE. 

"  Manufacture  of  Mineral  and  Lake  Pigments,"  J.  Bersch.     Translated 
by  Wright.     1901. 

"  Chemistry  of  Paints  and  Painting,"  A.  H.  Church.     1901, 
"  Manufacture  of  Paint,"  J.  C.  Smith.     1901, 


358  SILICA   AND   THE  SILICATES 

'  Chemistry  of  Pigments,"  E.  J.  Parry  and  J.  H.  Coste.     1902. 
'  Chemistry  of  Paint  and  Paint  Vehicles,"  C.  H.  Hall.     1906. 
'  Painters'  Colours,  Oils,  and  Varnishes,"  G.  H.  Hurst.     1906. 
'  Chemistry  and  Technology  of  Mixed  Paints,"  M.  Toch.     1907. 
'  Colour  Manufacture,"  G.  Kerr  and  R.  Rubencamp.     Translated   by 
C.  Mayer.     1908. 

"  Chemistry  of  Paints,"  J.  N.  Friend.     1910. 
"  Materials  of  the  Painters'  Craft,"  A.  P.  Laurie.     1910. 
"Pigments,"    "Paints,"   E.   G.   Clayton  in  Thorpe's  "Dictionary  of 
Applied  Chemistry." 

"  Rubber,  Resins,  Paints  and  Varnishes,"  Morrell  and  de  Waele.  This 
series.  1920. 

ADDITIONAL  LITERATURE. 

"  Chemistry  as  Applied  to  the  Arts  and  Manufactures,"  S.  Muspratt. 
London,  1860. 

"  Die  Feuerfesten  Thone,"  C.  Bischof.     Leipzig,  1876. 

"  Portland  Cement,"  H.  Reid.     London,  1877. 

Ure's  "  Dictionary  of  Arts,  Manufactures,  and  Mines."  7th  edition. 
London,  1878. 

'  Fabricant  de  Couleurs."     Paris,  1884. 

'  Bricks,  Tiles,  Terra  Cotta,  etc.,"  C.  F.  Davis.     Philadelphia,  1884. 

'  La  Porcelaine,"  Dubreuil.     Paris,  1885. 

'  La  Porcelaine,"  G.  Vogt.     Paris. 

'  Art  of  the  Old  English  Potter,"  L.  M.  Solon.     London. 

"  Constitution  of  the  Silicates,"  F.  W.  Clarke.  Washington,  1896  and 
1914. 

"  Chemical  Technology, "  R.  Wagner.  Translated  by  Crookes.  London, 
1897. 

"  Researches  on  Leadless  Glazes,"  W.  J.  Furnival.     Stone,  1898. 

"  Architectural  Pottery,"  L.  Lef£vre.  English  Translation.  London, 
1900. 

"  Notes  on  Pottery  Clays,"  J.  Fairie.     London,  1901. 

"  Leadless  Decorative  Tiles,  Faience,  and  Mosaic,"  W.  J.  Furnival. 
Stone,  1904. 

"  Clays :  Their  Occurrence,  Properties,  and  Uses,"  H.  Ries.  New  York, 
1906. 

"  Clayworkers'  Handbook,"  A.  B.  Searle.     London,  1906. 

"  Modern  Manufacture  of  Portland  Cement,"  P.  C.  H.  West.  London, 
1910. 

"  Materiaux  et  Produits  Refractaires,"  A.  Granger.     Paris,  1910. 

"  Modern  Brick-making,"  A.  B.  Searle.     London,  1911. 

"  British  Clays,  Shales,  and  Sands,"  A.  B.  Searle.     London,  1912. 

"  Mica,"  H.  S.  de  Schmid.     Ottawa,  1912. 

"  Handbuch  der  Mineralchemie,"  C.  Doelter  (Glas.  E.  Zschimmer). 
Dresden  and  Leipzig,  1912. 

"  Silicates  in  Chemistry  and  Commerce,"  W.  and  D.  Asch.  Translated 
by  Searle.  London,  1913. 

"  Clay  and  Pottery  Industries."  Collected  Papers  by  J.  W.  Mellor  and 
others.  London,  1914. 

"  Sands  Suitable  for  Glass-making,"  P.  G.  H.  Boswell.     London,  1916. 

"  Ceramic  Receipts,"  John  Bourne.     Hanley,  1884. 

"  Book  of  Modern  Recipes,"  W.  R.  Creyke.     Hanley,  1887. 

"  Chemical  Recipes  "  [W.  R.  Creyke  and  others].     Sunderland,  1896. 

"Rezeptbuch"  [Ceramic  Bodies,  Glazes,  Colours],  "Die  Glasshiitte." 
Dresden,  1907. 

"  Recipes  for  Flint  Glass-making,"  by  a  British  Glass  Master  and  Mixer, 
and  edition.  London. 


APPLICATIONS  OF  SILICATES  359 

For  further  references  the  following  may  be  consulted : — 

"  Bibliographic  Ceramique  Champfleury."     Paris,  1881. 

"  Bibliography  of  Clays  and  the  Ceramic  Arts,"  J.  C.  Branner.  U.S.A., 
1896  and  1906. 

"  Ceramic  Literature,"  M.  L.  Solon.     London,  1910. 

Also  Supplementary  List  of  Books  in  Trans.  Ceramic  Soc.,  II,  65,  1912. 

"  Bibliography  of  Glass,"  Solon.     Trans.  Ceramic  Soc.,  12,  65,  285, 1913. 

To  the  above  may  be  added  the  British,  American,  and  Foreign  Techni- 
cal Journals,  especially  those  concerned  with  pottery,  glass,  etc.  Among 
the  more  important  foreign  journals  may  be  mentioned  Sprechsaal  (Coburg), 
Keramische  Rundschau  (Berlin),  Tonindustrie  Zeitung  (Berlin),  La  Ceramique 
(Paris). 

Many  valuable  papers  will  be  found  in  the  Transactions  of  the  Ceramic 
Society,  the  Transactions  (Journal  from  1918  on)  of  the  American  Ceramic 
Society,  and  the  Journal  of  the  Society  of  Glass  Technology,  as  well  as  the 
Journal  of  the  Society  of  Chemical  Industry. 


INDEX 


AALBORG  kiln,  126 
Acheson,  E.  G.,  115 
Acid-proof  utensils,  4 
Acmite,  64 
Actinolite,  20,  65,  66 
Adhesive  cements,  166 
Adobes,  244 
Adularia,  91,  92 
^Egirite,  64 
Aerographing,  215 
Agalite,  68 
Agalmatolite,  74 

Agate,  16,  18,   20,   21,   22,   23,   95, 
120 

working,  22,  120 
Agate-opal,  39 
Ageing  of  prepared  clay,  190 
Aggregate  (for  concrete),  162 
Air-slaked  lime,  127 
Alabaster,  129 

glass,  314,  315 
Albite,  47,  48,  91,  92 
Alferite,  345 
Alite,  155,  156 
Alkali  for  glass,  285 
Alkali-lime  glass,  280,  281 
Alkaline  glazes,  215,  217 

substances  for  casting  slip,  193, 

194 

Allen's  Stag  tube-mill,  143 
Allen  and  Ames,  299 
Allophane,  73,  169 
Almandine,  90 
Alomite,  78 
Alum,  336-342 

cake,  344 

meal,  338,  339 

rock,  343 

schist,  337 

shale,  337,  338 

uses  of,  341 
Alumina,  268 

anhydrous  silicates  of,  70 

in  glass,  286,  314 

in  glazes,  217 
Aluminate  cements,  166 
Aluminite,  342 


Aluminium  silicates,  73 

sulphate,  338,  339,  341,  342-347 

sulphate  from  bauxite,  344-346 

sulphate  from  China  clay,  343, 

344 

Alumino-silicic  acids,  51,  72 
Aluminous  bricks,  258 

clays,  300 

Alumstone,  337,  343 
Alundum,  122,  268 
Alunite,  337,  343 
Alunogen,  342 
Amazon  stone,  92 
Amber  mica,  87 
Ambitty  sheet,  309 
American  natural  cement,  164 

Portland  cement,  159 
Amethyst,  18,  19,  90 
Amianthus,  66 
Ammonium  alum,  339,  342 
Amorphous  silica,  3,  6,  24,  34,  42,  45 

silica  and  silicates,  275 
Amphiboles,  63,  65-67,  71,  79,  89 
Anatase,  27,  29 
Andalusite,  27,  70,  71 
Andesine,  92 
Andesite,  33,  39,  105 
Andradite,  90 
Anhydrite,  129 
Anhydrous  silica,  3,  4,  38 
Annealing  temperature,  276 

of  glass,  276-278,  295-297 
Anorthite,  47,  48,  77,  85,  88,  91,  92 
Anthophyllite,  65,  71 
Antimony  oxide  in  glass,  313 
Apatite,  79 
Apophyllite,  49,  56 
Aquamarine,  71,  99,  100 
Argillaceous  sandstones,  29,  30 
Arkose,  29 

Armoured  concrete,  163 
Arnoux,  238 
Artificial  leucite,  94 

meerschaum,  135 

stone,  28,  61,  134 

ultramarine,  78,  79,  356 

vegetation,  61 


362 


INDEX 


Artificial  zeolites,  96 

Asbestos,  20,  60,  67,  69,  166,  300 

Asch,  W.  and  D.,  51,  157 

Ashcroft,  93 

Aspdin,  Joseph,  139 

Astbury,  Thomas,  181 

Augite,  21,  27,  64,  65,  67,  105 

Automatic  bottle-making  machines, 

307 

Avanturine  quartz,  20 
Aventurine  felspar,  92 

glass,  311 

quartz,  20 
Axinite,  88 
Azure  spar,  78 

stone,  78 

BACKS,  141 

Ball  clay,  76,  77,  175,  179,  207,  208 

clay  replaced  by  fireclay,  176 
Banded  agate,  20 

jasper,  24 
Banks,  E.,  201 
Barbotine  painting,  226,  241 
Barium  disilicate,  310 
Barnitt,  J.  B.,  165 
Barytes,  30 
Basalt,  105 

for  making  glass,  284,  306 
Basanite,  25 

Basse-taille  enamelling,  324 
Bastite,  64 
Batch  stones,  311 
Bath  bricks,  28 
Batting  machines,  193,  196 
Bauxite,  76,  268,  344,  345 

saggars  from,  208 
Bayer,  346 
Behrens,  39 
Belite,  155 
Benrath,  280,  281 
Benson,  Thomas,  180 
Bentley,  181 
Beryl,  71,  99,  100 
Berzelius,  1 

Beth  dust  collector,  152 
Beton,  162 

Bigot,  Dr.  A.,  265,  266 
Biotite,  84,  85 
Biscuit   firing,    201,    203,    209-211, 

224,  236,  237,  238 
Bismuth  oxide  in  glass,  308 
Black  basalt,  233 

chalk,  355 

glass,  314 

opal,  40 
Blast  furnace  slags,  62,  111 

furnace  slags  for  making  glass, 
289,  306 


Bleininger,  A.  V.,  261,  299 
Bleriot,  269 
Block  alum,  339 
Bloodstone,  20,  23 
Blow-iron,  308 
Blowing,  215 
Blue  asbestos,  66 

bricks,  210,  254 

stain  for  bodies,  185 
Blunger,  184 
Bock,  L.,  357 

Bodies,  classification  of,  178 
Body  and  glaze,  composition  of,  220 
Boetius  furnace,  291,  292 
Bogitch,  B.,  265 
Bohemian  garnets,  90 

glass,  279,  305 
Boiling  gypsum,  102 
Bole,  352 
Boiling,  116 
Bone,  grinding  of,  183 

porcelain,  213,  233-235,  237 
Borate  flint  glass,  319 

glass,  280 
Boric  acid,  3 
Borosilicate  crown  glass,  319 

crown  glass,  279 
Bostock,  E.  H.,  304 
Boswell,  P.  G.  H.,  283,  284 
Bottle  glass,  279,  283,  284,  305-307 

glass  from  slag,  111,  112,  306 

kiln,  147 

Bowen,  N.  L.,  311 
Bowenite,  69 
Boys,  Prof.  C.  V.,  43 
Bradley  three-roll  mill,  140 
Branson,  F.  W.,  333 
Brazilian  emerald,  100 
Bredel,  44 

Brick  earths,  77,  244,  245 
Brick  kilns,  252-254 
Brick-press,  250 
Bricks,  178,  210,  243-255 

for  insulating,  107 
Bristol  diamond,  19 
British  developments,  270,  332 

Hartford-Fairmont   Syndicate, 

307 

Britten,  B.,  Ill,  306 
Broad  glass,  303 
Bronzite,  63,  64 
Brown,  G.  H.,  206 

R.  E.,  329 

Brown  ochre,  351,  352,  354 
Bubbles  in  clay  slip,  curious  effect 
of,  184 

or  seed  in  glass,  317,  318 
Buhrstone,  23,  25 
Buller's  rings,  211 


INDEX 


363 


Bullion  or  Bull's-eye,  303 
Burnt  alum,  340 

ochre,  352 

sienna,  352 

umber,  354 
Burrstone,  25 
Butler,  D.  B.,  152 
Bytownite,  92 

CACHOLONG,  40 

Cadmium  sulphide  in  glass,  313 
Cairngorm,  18,  19 
Calcareous  sandstone,  29,  30 
Calcination  of  flint,  180,  181 
Calcium  carbonate,  62 

fluoride,  314 

metasilicate,  62,  310 

orthosilicate,  63 

permutite,  98 

phosphate,  314,  347 

sulphate,  129-132 
Caledonian  brown,  354 
Californite,  100 
Calorites,  211 
Cancrinite,  78 
Candlot,  54 
Canning,  H.,  306 
Cape  asbestos,  66 
Cappagh  brown,  354 
Carbolon,  117 
Carbon  in  glass,  312 
Carborundum,  44,  115-119,  122,  208 
Carbuncle,  90 
Carnelian,  20,  22,  23 

onyx,  23 
Carrara,  237 

Cassel  brown  or  Cassel  earth,  355 
Cast-iron  enamels,  326,  327 
Cast  ware,  contraction  of,  200 
Casting  ceramic  wares,  190,  193-195 

glasshouse  pots,  299 
Casts  (plaster,  etc.),  133,  134 
Cat's-eye  quartz,  20 
Celite,  155,  156 
Celadonite,  21 

Cement,  54,  60,  123  (see  also  Port- 
land cement,  Roman  cement) 

kilns,  53,  54,  147 

mortar,  137 

Ceramic,  meaning  of  the  word,  168, 
169 

bodies,  classification  of ,  178,179 

glazes,  92 

industries,  168 

products,  71 
Chabazite,  95 
Chalcedony,  6,  16-18,  20-26,  29,  34, 

40 
Champion,  Richard,  177,  238 


Champleve  enamelling,  324 
Chance,  K.  M.,  94 

Messrs.,  332 
Chaux  de  Theil,  129 
Chemical  glass,  279,  280,  283,  319 

stoneware,  232 
Chert,  25,  27,  182 
Cherty  sandstones,  29 
Chestnut  brown,  354 
Chiastolite,  70 

China  clay,  75,  82,  83,  104,  175,  176, 
179,  206,  207,  208 

clay  as  pigment,  355 

stone,  104,  176 
Chinese  white,  356 

yellow,  351 
Chlorite,  27,  29,  87,  101,  105 

schist,  108 
Chlorophaeite,  21 
Chocolate  whetstone,  31 
Chrome  bricks,  267,  268 

green,  62 

mica,  87 
Chromite,  27 

bricks,  267,  268 
Chromium  aventurine  glass,  31 1 

oxide  in  glass,  313 
Chroustschoff,  34,  35 
Chrysoberyl,  27 
Chrysolite,  52,  71,  100 
Chrysoprase,  20,  22,  23,  95 
Chrysotile,  6,  66,  69 
Cingalese  garnet,  90 
Cinnamon  stone,  90,  91 
Citrine,  18,  19 
Clamp  firing,  251 
Clarke,  F.  W.,  3,  51 
Clarke's  mill,  142 
Clay,  26,  29,  61,  72,  75-77,  166 

in  all  ceramic  wares,  169 

rocks,  29 

slate,  70 

Clayey  sandstones,  29 
Clays,  blunging  of,  183 

general  and  special  characters, 

170 

Cloisonne  enamelling,  324 
Cloudiness    produced    by    fluorine, 

331,  332 

Coagulation  of  hydrated  silica,  37 
Coal  brown,  355 
Cobalt  oxide  in  glass,  313 
Coe,  J.  H.,  326 

Colburn  process  for  sheet  glass,  304 
Colcothar,^53,  354 
Colloidal  alumina,  73 

silica,  20,  24,  34,  38,  55 

solutions,  4,  6,  37 
Cologne  earth,  355 


364 


INDEX 


Colour  changes,  210 
Coloured  glasses,  315 

glazes,  223 

slips,  226 

Colouring  oxides,  227,  228 
Combining    weights     for     ceramic 

materials,  219 
Common  opal,  39 

pottery,  240 
Compo,  145 

Concentrated  alum,  345 
Concrete,  14,  162,  163 
Conglomerates,  29 
Constitution  of  mixed  silicates,  48 
Contraction,  173,  174,  199,  200,  209, 

210,  220 
Cookworthy,    William,     176,     177, 

238 

Copeland,  237 
Copper-red  colour,  225,  229 
Copper  silicate,  357 
Coral  sands,  27 
Cordierite,  101 
Corindite,  268 
Cornelian,  23 
Cornish  diamond,  19 

stone,  104,  176,  177 
Corundum,  71,  100,  208 
Cotterite,  19 

Covered  pots  (for  glass),  297,  300 
Cracking  of  earthenware,  34,  175 
Crampton,  147 
Crazing,  180,  220,  221 
Cristobalite,  6,33-36,  180,  260,261, 

262,  310,  311 
Crocidolite,  20,  66,  67 
Crocus,  353 
Crown  drops,  311 

glass,  279,  302,  305,  319 
Crucibles,  258 
Cryolite,  314 
Crystal,  11 

glass,  279,  307,  308,  322 
Crystalline  glazes,  223,  225 
Crystallites,  106,  107 
Crystallization,  274,  275 
Crystallized  silica,  3,  5,  6,  9,  14,  32, 

35,  43,  45 
Crystolon,  117 
Culinary  stoneware,  232 
Cullen  earth,  355 
Cullet,  286 

Cummer  rotary  calciner,  132 
Curtius,  83 
Cutler's  cement,  167 
Cutting  precious  stones,  121 
Cuttings,  186,  190 
Cyanite,  70 
Cyclops,  21 


DANIELSON,  R.  R.,  328 
Daubree,  35 

Davidson,  J.  H.,  302,  307 
Day,  35,  36,  158 
Decolorizing  glass,  287-289 
Decoration,  224 

in  the  clay,  198,  199 
Decorative  tiles,  241 
Delessite,  21 

Dennis  simplex  furnace,  295 
Depression  temperature,  276 
De  Senarmont,  35 
Desert  sand,  26,  28 
Devitrification,    44,    62,    217,    275, 

309-311,  314,  318 
Devitrified  glass,  46,  6-1,  309 
Devonshire  batts,  32 
Diabase,  52 
Diamond,  27,  31,  71,  121,  122 

dust,  121,  122 

paste,  309 

Diatomaceous  earth,  41 
Diatomite,  41 
Dicalcium  silicate,  53 
Dichroite,  101 
Dietzsch  kiln,  126,  147 
Dimetasilicic  acid,  50 
Dinas  bricks,  264,  266 
Diopside,  64,  65,  79 
Diorite,  104 
Diorthosilicic  acid,  50 
Dipping,  213 
Dispersion,  320 
Disthene,  70 
Ditter,  156 
Dolerite,  52,  105 
Dolomite,  30,  267 
Dolomitic  refractories,  267 
Double  soluble  glass,  59 
Down-draught  ovens,  201,  203 
Drain  pipes,  232 
Drummond,  269 
Dry  mixing,  185 
Drying  bricks,  250,  251 

clay,  174,  199 

clay  slip,  187 

glazed  ware,  223 

stove,  194,  197,  198 
Dumesnil's  kiln,  131 
Dumont's  blue,  356 
Dunnachie's  kiln,  254 
Duroglass,  319 
Dusting,  215 
Dutch  tears,  297 
D wight,  John,  181,  234 
Dynamite,  41 

EARTHENWARE,  34,  178,  179-230 
Earth's  crust,  3 


INDEX 


365 


Earthy  gypsum,  129 

pigments,  350-355 
Egyptian  black,  233 

blue,  357 

jasper,  24 
Einthoven,  43 
Eisenkiesel,  20 
Elaeolite,  78,  104 

syenite,  104 
Electric  calamine,  56 
Electro-osmosis  treatment  of  clays, 

176 

Eleod,  K.,  208 
Emerald,  100 
Emery,  14,  117,  118,  122 
Enamel,  212,  215,  224,  273,  322 

colours,  229 

powders,  330,  331 

printing,  227 
Enamelled  cast  iron,  326,  327 

sheet  iron,  325,  326,  328 
Enamelling  on  metal,  322-332 
Enamels,  fusibility  of,  328,  329 

refractoriness  of,  327 
Engels,  118 
English  crucibles,  258 

porcelain,  213,  233-235 
English,  S.,  277 
Engobe,  259 
Enstatite,  27,  50,  63,  64 
Epidote,  29,  88,  89 
Eruptive  rocks,  20 
Essonite,  90,  102 
Estrichgips,  132,  133 
Etching  glass,  9 
Euchrome,  354 
Euclase,  100 
Evening  emerald,  100 
Expansion  coefficient,  13,  14,  45 
Expansion  of  silica,  45 

on  heating,  13,  16,  17,  18,  175 
Eye-agate,  21 

FABRONI,  255 
Faience,  178 
Fat  clays,  246 

lime,  126,  136 
Fayalite,  50,  52 
Feather  alum,  342 
Felite,  155,  156 
Felsite,  27 
Felspathic  glazes,  215 

sandstone,  30 

Felspar,  27,  29,  46,  47,  48,  50,  75,  77, 
78,  79,  89,  91,  92,  95,  100, 
101,  103,  104,  105,  176,  177, 
183 

etc.,  as  sources  of  potash,  92-94 
grinding  of,  183 


Fenner,  C.  N.,  319,  321 
Ferguson,  33,  319 
Ferro-concrete,  163 
Ferrosilicon,  112,  113,  118 

alloys,  114 

Ferruginous  quartz,  20 
sandstones,  29,  30 
Fibrolite,  70 
Fibrox,  119 
Filter  press,  187,  188 
Filtration  of  water,  28 
Fine  earthenware,  178 

stoneware,  178,  232,  233 
Fiorite,  40 
Fire  polishing,  303 
Firebricks,    71,    74,    178,    256-258, 

264 
Fireclay,  76,  77,  176,  204,  205,  208, 

257,  289 

replacing  ball  clay,  176 
ware,  258,  259 
Fire  garnet,  90 
Fire  opal,  39 
Fireproof  bricks,  42 
Firing  bricks,  251 
effects  of,  209 
glazed  ware,  223 
ware,  200-212,  223 
Fish-eye  stone,  56 
Fisher,  A.,  325 
E.  E.,  302 
Fissot,  134 
Flagstone,  30 
Flare  kiln,  125 
Flashing  furnace,  303 
Flat  and  hollow  ware,  196 
Flint,  7,  17,  23,  24,  25,  34,  58,  59,  64, 

77,  180-182 
effects  of,  on  body,  180 
glass,  279,  307,  316,  319 
glass  for  enamels,  323 
Flinty  slate,  25 
Floating  bricks,  255 
Flooring  plaster,  132,  133 

tiles,  178 
Flows,  214 
Fluor  spar,  100,  314 
Fluorine,   cloudiness  produced   by, 

331,  332 
Fluorite,  30 

Fluxing  materials,  178-180 
Fontainebleau  sandstone,  30 
Forester,  T.,  201 
Forsterite,  52 
Fortification  agate,  21 
Fouque,  33,  35,  357 
Fournet,  111 
Frazer,  94 
Freestone,  30 


366 


INDEX 


Friedel,  35 
French  blue,  356 

soft  porcelain,  235 
Frink,  R.  L.,  282 
Fritted  glazes,  216,  218 

porcelain,  235 
Fritting,  216 
Frommel,  W.,  288 
Frost,  L.  J.,  329 
Frye,  Thomas,  234 
Fuchsite,  87 
Fuller,  D.  H.,  299 
Fuller's  earth,  68,  74 
Furnace  blocks,  289 

linings,  178 
Fused  quartz,  42 

silica,  43-45 
Fusibility,  175,  222 

of  enamels,  328,  329 

of  glaze,  222 

range  of  industrial  glasses,  277 

GABBRO,  62 

Galvanometer  filaments,  43 

Ganister  bricks,  264 

Canisters,  29,  30 

Gans,  96,  98 

Garnet,  27,  29,  31,  49,  89-91,  95,  103, 

118,  120 

Gas-fired  ovens,  204 
Gaudin,  42 
Gelatinous  silica,  6,  37,  38,  52,  57, 

73,  95,  110,  169 
Geller,  R.  F.,  170 
Gem  sands  and  gravels,  27 
Gem  cutting,  119 
Geode,  21,  36 
Geolyte,  95 
Geyserite,  40 
Gilding,  etc.,  230 
Girasol,  39 

Glass,  3,  8,  9,  10,  25,  28,  46,  62,  107, 
110,  273-321 

from  blast  furnace  slag,    111, 
112,  306 

furnaces,  289-297 
Glass-gall,  308 
Glass-making,  273 
Glass,  principal  types  of,  302-309 

properties  of,  281,  282 
Glasses,  classification  of,  278-280 

composition  of,  217 
Glasshouse  pots,  297-300 
Glassy  felspar,  91,  92 

opal,  38 

Glauconite,  6,  27,  29,  30,  101,  102, 
172 

as  a  source  of  potash,  102 
Glaze,  essential  qualities  of,  212 


Glazes,  calculations  for,  218 

classification  of,  215 

composition  of,  216-220 
Glazing  of  earthenware,  etc.,  212- 

215 

Glost  placing  and  firing,  224,  236 
Gmelin,  79 
Gmelin's  blue,  356 
Gneiss,  107,  108 
Gold,  27 

colours,  229 

ruby  glass,  311,  312 
Golden  ochre,  351 
Goreham  process,  141 
Graham,  265 
Granger,  Dr.  A.,  228 
Granite,  4,  61,  102-104,  182,  306 
Granitic  rocks,  14,  30,  33 
Graphite  crucibles,  258 
Grappier  cement,  129 
Gravel,  26,  27,  28,  61 
Green,  Guy,  226 
Green  earth,  353 

sands,  27,  29,  102 

sandstones,  29 

ultramarine,  80,  356 
Grenet,  276 
Greywackes,  29 
Griffin  mill,  143,  146,  150 
Grinding,  179 

cylinder  for  flints,  etc.,  182 

flint,  182 

plant  for  flint,  etc.,  181,  182 
Grindstones,  30 
Grisaille  painted  enamels,  324 
Grit,  29,  30 
Grog,  204,  205,  206,  208,  257,  289, 

298,  299 
Grossularite,  90 
Ground  coat  enamel,  326-328,  331, 

332 

Grout  (grouting),  137,  163 
Guimet,  79 
Guimet's  blue,  356 
Gypsum,  77,  123,  129-132 

HEMATITE,  23,  29,  30 

Hair  salt,  342 

Hair-stone,  20 

Halloysite,  74,  75 

Halotrichite,  342 

Handles,  194,  195 

Hard  porcelain,  212,  237-239,  270 

Hardening   of   lime,  cements,    etc., 

55,  127,  134,  136,  159 
Hardening-on,  226,  227 
Hard-finish  plasters,  135 
Harlequin  opal,  39 
Hart,  93 


INDEX 


367 


Hartford-Fairmont  Feeder,  307 

Hautefeuille,  36 

Haiiy,  63 

Hauyne,  78,  79,  80 

Hauynite,  78 

Hawk's  eye,  20,  67 

Heath,  181 

Kecking,  131 

Hecla  disc  crusher,  142 

Heinecke,  205 

Heliotrope,  20,  23 

Henning,  45 

Heraeus,  44 

Hermansen  furnace,  292,  293,  333 

Herschkowitsch,  36,  44 

Hessian  crucibles,  258 

Heulandite,  95 

Heylyn,  234 

Hiddenite,  64,  100 

Hindostan  whetstone,  31 

Hob-mouthed  ovens,  200 

Hodkin,  308 

Hoffmann,  253 

kiln,  126,  147,  253 
Holborn,  45 

Holdcroft's  thermoscope  bars,  211 
Holland,  94 
Hollow  ware,  196 
Homberg's  phosphorus,  340 
Hone,  93 
Hones,  31 
Hone-stone,  25,  31 
Hornblende,  27,  67,  103,  105 

schist,  108 
Hornstone,  25 

slate,  25 
Hovel,  200,  203 

ovens,  200 
Hunt,  R.,  306 
Hurry,  E.  H.,  148 
Hyacinth,  91,  100,  102 
Hyalite,  38 
Hyalosiderite,  52 
Hydrargillite,  169 
Hydrated  compounds  of  silica  and 
alumina,  72 

lime,  128 

magnesium  silicates,  67,  69 

silica,  3,  6,  7,  8,  16,  37,  38,  42 

silicate  of  lime,  55 
Hydraulic  cements,  55,  123,  166 

limes,  8, 53,54,  111,  123,128, 129 

mixtures,  123 

mortar,    106,    136,    137,    139, 
162 

properties  of  ultramarine  blue, 

80 

Hydrophane,  40 
Hypersthene,  63,  64,  105 


ICE-SPAR,  91 

Idocrase,  100 

Imitation  gems,  321,  322 

meerschaum,  42 
Immersion,  213 
Impermeable  pottery,  179 
Inclusions  in  quartz,  13 
Indian  red,  352,  353 

ochre,  352 
Indicolite,  88 

Indigo  blue  tourmaline,  88 
Infusorial  earth,  41,  58,  59,  69 
lolite,  101 
Iron  content  in  glass,  284 

flint,  20 

pyrites,  30,  186,  210 

red  colour,  229 

speck,  186,  209 
Isle  of  Wight  diamond,  19 
Isomorphous  mixtures,  47,  89,  95 
Italian  earth,  352 


JACINTH,  102 
Jackson,  Sir  H.,  333 
Jade,  64,  66,  69,  100 
Jadeite,  64,  66,  69,  73 
Janecke,  157 
Jargon  (jargoon),  102 
Jasper,  21,  23,  24 
body,  179,  233 
-opal,  39 

'ersey  stone,  177 

[esser,  156 

ewelled  bearings  for  watches,  122 

igger,  197 

ob's  tears,  52 

ohns,  Cosmo,  264 

ohnson,  J.  E.,  112 

olleys,  196,  197 

KALINITE,  337 

Kaolin,  27,  30,  60,  71,  75,  76,  77,  99 

as  pigment,  355 
Kaolinite,  71,  72,  74,  75 
Kappen,  H.,  156 
Keene's  cement,  132 
Keiserman,  156 
Kentish  rag,  30 
Kersten,  111 
Kieselguhr,  40-42 
King,  P.  E.,  98 
Kirkpatrick,  F.  A.,  206,  312 
Kite,  James,  187 
Koksharovite,  79 
Kornerupine,  49,  65 
Kottig,  79 
Kraze,  F.,  330,  331 


368 


INDEX 


Kritzer  hydrator,  128 
Kunz,  G.  F.,  120 
Kyanite,  27,  70,  71 

LA  FAROE  cement,  129 

Labradorite,  92 

Lacroix,  34,  35,  36 

Laird,  J.  S.,  170 

Landrum,  R.  D.,  329 

Lapidary's  work,  119,  121 

Lapis  lazuli,  22,  78,  79 

Laurie,  357 

Lava,  4,  33,  92,  104,  105,  106 

Lawns,  186 

Lazulite,  78 

Lazurite,  79 

Lead  glazes,  215,  217,  218 

silicates,  309 
Leadless  enamels,  60 

glazes,  217 
Lean  lime,  126 
Lecesne,  N.,  268 

Le  Chatelier,  H.,  35,  36,  38,  45,  53, 
55,  56,  63,  71,  154,  155,  157, 
159,  171,  172,  262,  263,  265, 
276,  280,  281,  344 
Lehr,  295-297 
Lepidolite,  84,  85,  87 
Le  Roy,  3 
Leucite,  73,  78,  101,  105 

(artificial),  94 

as  a  source  of  potash,  91 
Leucones,  5 
Levant  umber,  353 
Levy,  33,  35 
Licht,  253 
Light  red,  352 
Lignite,  186 
Lime,  123-128 

for  glass,  285,  286 

in  silica  bricks,  260 

-burning,  124-126 

felspar,  77,  92 

glazes,  217,  218 

-kilns,  125,  126 

-mortar,  62 

Limestone,  25,  62,  77,  79,  88,  90 
Lime-water,  127 
Limonite,  20,  21,  29,  30,  102 
Lining  for  furnaces,  118 
Liquid  inclusions,  13 
Literature,  45,  122,  167,  272,  334, 

335,  357,  358,  359 
Lithia  emerald,  100 

mica,  87 

Lithography,  226,  235 
Lithoxylon,  40 
Liver  opal,  40 
Loam,  26 


Louisville  cement,  164 

Lucas,  Thomas,  226 

Lustre  decoration,  230 

Lutecite,  23 

Lydian  stone,  23,  25 

Lydite,  25 

Lynch  bottle- making  machines,  307 

MACHINE  for  brick-making,  249 

-made  bricks,  249 
Mack's  cement,  135 
McLintock,  357 
Magic  stone,  40 
Magnesia  bricks,  267 

crystals   for   optical   purposes, 

321 
Magnesian  silicates,   7,  64,  65.  67, 

69 
Magnesite,  68,  266 

bricks,  267 

Magnesium  metasilicate,  63 
Magnetic  oxide,  264,  265 
Magnets,  187 
Malinovszky,  A.,  268 
Mallard,  33 
Malm,  247 
Manganese  blue,  356 

in  glass,  286,  313 

-permutite,  98 

silicate,  63 

Mannheim  calciner,  132 
Marble  cement,  134 
Marbles,  103 
Marls,  77,  245,  247,  248 
Mars  brown,  352 

orange,  352 

red,  352 

violet,  352 

yellow,  352 
Martin's  cement,  135 
Massive  gypsum,  129 
Marvering  glass,  302 
Meerschaum,  67,  68,  69 

(imitation),  42,  69 
Melanite,  90 
Mellor,  Dr.  J.  W.,  72,  75,  214,  234, 

235,  238,  270 
Mendheim's  kiln,  254 
Mendozite,  341 
Menilite,  40 
Merian,  32 

Merton,  Dr.  T.  R.,  333 
Merwin,  33 
Metal  cement,  166 

moulds,  193 
Metasilicates,  57 
Metasilicic  acid,  50 
Meteoric  stones,  63 
Meteorites,  33,  52 


INDEX 


369 


Mica,  20,  27,  76,  84-87,  103,  172 

for  paints,  86 

schist,  31,  65,  86,  108 

slate,  70 

Micaceous  sandstones,  29,  30 
Micanite,  86,  87 

cloth,  86 

Michaelis,  Dr.  W.,  153,  256 
Microcline,  91,  92 
Microlites,  106,  107 
Miles,  357 
Milk  of  lime,  127 

opal,  39 

quartz,  18,  19 
Milkiness  in  glass,  319 
Millar  bottle-making  machine,  307 
Miller,  94 
Millstone  grit,  30 
Millstones,  14,  25,  30 
Mineral  black,  355 

brown,  354 

yellow,  351 

Miniature  enamel  painting,  325 
Minneman,  J.,  323 
Minton,  Herbert,  238 

T.  W.,  202 
Mintons,  238 

Minton's  down-draught  oven,  202 
Mirrors,  304,  305 
Mitscherlich,  47 
Mitscherlich's  law,  47 
Mixed  cements,  124,  163 

silicates,  48 
Mixing  earthenware  body,  179,  185 

glass  batch,  286,  287 
Mocha  stone,  21,  22 
Moissan,  2,  43 
Moler  bricks,  255 
Monazite,  321 
Monox,  4,  355 
Montgomery,  R.  J.,  321 
Monticellite,  52 
Moonstone,  91,  92 
Moore,  Bernard,  235,  238,  270 
Morey,  G.  W.,  321 
Morion,  19 
Mortar,  111,  123,  135-137 

bodies,  178,  179,  233 

bricks,  255 
Moss  agate,  21 
Mother  of  emerald,  100 

of  opal,  39 
Mottram,  263 
Moulding  sands,  28 
Moulds  for  pressing,  193 
Mud,  26,  27,  29 
Muller's  glass,  38 
Murchisonite,  91 
Muscovite,  49,  74,  84,  85,  86,  87 

C. 


NATROLITE,  89,  95 
Natural  cements,  54,  164 

Portland  cements,  139,  164 
Needham,  William,  187 
Needle  stone,  20 
Nehse-Dralle  furnace,  292 
Nepheline,  78,  95 
Nephelite,  77,  78,  101,  105 
Nephrite,  64..  66,  69 
Nernst,  269 
Neutral  alum,  340 
New  blue,  356 

process  lime,  128 
Newberry,  S.  B.  and  W.  B.,  158 
Newell's  ball-mill,  151 

rotary  cement  kiln,  148 
Newtonite,  174 
Nickel  oxide  in  glass,  313 
Nitro-glycerine,  41 
Noble  garnet,  90 

opal,  38,  39 

serpentine,  69 
Non-aluminous  silicates,  51 
Non-plastic  materials,  178,  180 
Novaculite,  31 

OBSIDIAN,  106 

Occidental  topaz,  19,  71 

Ochres,  76,  351,  352 

Oden,  Sven,  169 

Oil  black,  355 

Oils  for  printing,  226 

Oilstone,  18,  311 

Oligoclase,  92 

Olivine,  27,  50,  51,  63,  69,  71,  105 

O'Neill  bottle-making  machines,  307 

On-glaze  colours,  229,  236 

decoration,  224 
Onyx,  20,  22,  23 
Opal,  17,  38-40,  41 

glasses,  310,  314 
Opalization,  309,  310 
Opaque  glass,  314 

glazes,  215 

white,  228 
Open  pots,  298 
Open-burning  clays,  206 
Optical  glass,  279,  283,  316-321 

23  types  of,  321 

with  cerium,  321 
Oriental  amethyst,  90 

blue,  356 

emerald,  100 

garnets,  90 

topaz,  71 

Orthoclase,  47,  79,  91,  92,  177 
Orthosilicates,  51,  53,  56 
Orthosilicic  acid,  50 
Ostwald,  341 

24 


370 


INDEX 


Oval  dishes,  etc.,  195 
Ovens  for  pottery,  etc.,  200-204 
Owens  bottle-making  machines,  307 
Oxford  ochre,  351 

PAGODITE,  74 

Painted  enamels,  324,  325 

Painting  glass,  315 

glaze,  215 

with  slip,  226 
Paragonite,  85 
Parian,  179,  236,  237 

cement,  134 
Parker,  James,  138 
Parker's  cement,  138 
Parkes,  181 
Parmentier,  35 
Pasley,  General,  139,  141 
Passow  cement,  164,  165 
Paste,  321,  322 
Paulite,  63 
Paving  tiles,  242,  243 
Pealite,  40 
Pearl  sinter,  40 
Pearls,  transparent,  46 
Pebble,  19 

Peddle,  Dr.  C.  J.,  281,  285,  310,  334 
Peeling  of  glaze,  180,  220,  222 
Pegmatite,  86,  104 
Peridot  (peridote),  52,  100 
Peridotites,  52 
Perin's  barrel  kiln,  131 
Perlitic  structure,  106 
Permanent  blue,  356 
Permeable  pottery,  179 
Permutite,  96-98 

process  for  softening  water,  96 
Persian  red,  352 
Petinot,  N.  G.,  114 
Petrosilex,  4 

Petry  and  Hecking's  kiln,  131 
Phenacite  (phenakite),  56 
Philipon,  263 
Phlogopite,  84,  85,  86,  87 
Phosphate  crown  glass,  319 

glass,  280 

of  silica,  9,  10,  35 
Phosphatic  glass,  280 
Pigment,  4,  5,  350-357 
Pinite,  74,  101 
Pitcher  moulds,  193 
Pitchstone,  107 
Placing,  208,  209,  223 
Plasma,  20,  23 
Plaster  casts,  133,  134 

cements,  135 

moulds,  193 

moulds  for  drying,  187 

of  Paris,  130,  133,  134,  166 


Plaster  stone,  129 
Plastic  cements,  165-167 

materials,  178,  179 

silica,  44 
Plasticity  of  clays,  170-173,  186,  207 

of  ground  mica  and  glauconite, 

172 

Plate  glass,  304,  318 
Platinum,  27 

Plique-a-jour  enamelling,  324 
Plumbago  crucibles,  258 

pots,  300 

Pochin's  patents,  343 
Podzsus,  269 
Polarization,  12,  14-16 
Pontil,  303,  308 
Poor  lime,  126 

Porcelain,  17,  68,  71,  74,  178,  179 
Porosity,  174,  206 
Porphyry,  4,  103 

Portland  cement,  54,   93,  94,   123, 
139-160,  165 

British  Standard  Specification 
for,  153,  154 

composition  of,  152-154 

constitution  of,  154-160 
Poste,  E.  B.,  329 
Pot  arch,  299,  300 

stones,  31 
Potash  alum,  336-340 

felspar,  91 

from  felspar,  etc.,  92-94 

from  glauconite,  102 

from  leucite,  101 

lead  glass,  279 

lime  glass,  279 
Potassium  alum,  336-340 

silicate,  59,  60 
Potato-stone,  19,  20 
Potter,  116 
Potter's  clays,  240 

process,  4 

wheel,  190,  191 
Pottery,  66,  75,  178 

clays,  82 
Pounce,  107 
Pozzuolana,  160,  161 

cements,  124,  160,  161 
Prase,  18,  20 
Prase-opal,  39 
Precious  garnet,  90 

opal,  39 

serpentine,  69 
Prehnite,  95 
Press  cloths,  188,  189 
Pressed  glass,  305 
Pressing,  190,  192,  193,  195 
Princess  Blue,  78 
Printed  enamels,  226 


INDEX 


Printing  on  pottery,  etc.,  226 
Protecting  porous  stones,  61 
Pseudo-chalcedonite,  23 
Pseudo-wollastonite,  55,  62 
Pug-mill,  189,  190,  250 
Pulverizing  mill,  181 
Pumice,  92,  107,  161 
Pump  for  clay  slip,  187 
Purple  of  Cassius,  229 

stone,  176 
Putty,  166 
Pyro- emerald,  100 
Pyrometers,  211 
Pyrometric  cones,  211 
Pyrope,  90 
Pyrophyllite,  73,  74 
Pyrite(s),  30,  77,  79,  210 
Pyroxenes,  47,  49,  62-65,  67,  69,  79, 
89 

QUARTZ,  5,  7,  11-16,  17,  18-20,  26, 
35,  36,  103,  176 

crystals,  11,  12,  13,  16,  20,  35 

glass,  43-45,  280 
Quartzine,  23 

Quartzite,  14,  18,  31,  259,  260 
Quicklime,  126,  127,  128 

RAMANN,  E.,  98 
Rammelsberg,  48 
Rand,  C.  C.,  320 
Randanite,  41 
Rankin,  157 
Ransome,  147 
Raw  flour,  145 

glazes,  215 

materials  for  glass,  282 

meal,  145 

sienna,  352 

umber,  353 

Reaumur  porcelain,  235,  309 
Rectorite,  74 
Red  bricks,  245 

chalk,  352 

crystals,  224 

glass,  312 

haematite,  352,  353 

ochre,  352 

tourmaline,  88 

ultramarine,  80 
Refractive  index,  319,  320 
Refractory  bricks  (silicon  carbide), 
118 

cement,  118 

clay  products,  178 

clays,  177,  205,  289 

products,  33 

Reinforced  concrete,  163 
Rengade,  265 


Resinite,  40 
Resin-opal,  39 
Retorts  for  zinc,  118 
Rhodolite,  91 
Rhodonite,  63 
Rhyolite,  105 
Riband  agate,  20 

jasper,  24 
Rich  lime,  126 
Richardson,  C.,  156 
Riddle,  F.  H.,  299,  326 
Ring  agate,  21 
Robbia,  della,  325 
Roberts,  G.  G.,  312 

H.  S.,  321 
Robey,  C.,  201 

Robey's  down-draught  oven,  201 
Robinson,  James,  226 
Rocaille  flux,  309,  316 
Rock  crystal,  11,  18,  19,  71 
Roman  alum,  337,  340 

cement,  123,  138,  139 

ochre,  351 

yellow,  351 
Romanzowite,  91 
Roofing  slates,  109 

tiles,  178,  242 
Root-of-opal,  39 
Rose,  G.,  35 
Rose-opal,  39 
Rose  quartz,  18,  19 
Roseki,  74 

Rosendale  cement,  164 
Rosenhain,  Dr.  W.,  320,  333 
Ross,  D.  W.,  261,  262 

W.  H.,  92 

Rotary  kilns,  54,  126,  147-150 
Rottenstone,  122 
Rotting  of  prepared  clay,  190 
Rouge,  68,  122,  353 
Royal  blue,  356 
Rubellite,  88 
Rubens  brown,  355 
Ruby,  27 
Ruddle,  352 
Rudersdorf  kiln,  126 
Rumford's  kiln,  126 
Running  kilns,  125 
Rupert's  drops,  297 
Rutile,  20,  27,  29,  31 

SADLER,  John,  226 
Saggar  marls,  204,  206 

mending,  208 
Saggars,  204-208,  238 
Salt  glaze,  9,  212,  214,  232 
Salvetat,  41,  223 
Sand,  14,  26-28,  44 

blast,  14,  27,  28 


372 


INDEX 


Sand  dunes,  26 

for  glass-making,  283-285 

-lime  bricks,  7,  28,  256 

rock,  29 
Sanders,  269 
Sandiver,  308 
Sandstone,  14,  28-31 
Sanidine,  91 
Santorin  earth,  161 
Sapphire  d'eau,  101 

quartz,  18,  20 
Sard,  20,  23 
Sardonyx,  20,  22,  23 
Satin-spar,  129 
Saxon  blue,  356 
Scanegatty's  kiln,  131 
Scarlet  ochre,  352 
Schaefer,  J.,  329 
Schiller-spar,  64 

Schists,  4,  31,  64,  70,  71,  88,  90,  108 
Schmatolla's  kiln,  126 
Scholes,  93 
Schorl,  88,  104 
Schumann,  157 
Schuster,  32 
Scoriae,  111 
Scotch  pebbles,  21 

topaz,  19,  71 
Scott's  cement,  135 
Seaman,  H.  J.,  148 
Seaver,  261 
Seed  in  glass,  317 
Seger,  H.  A.,  216,  220,  221,  222 

cones,  211,  270 

porcelain,  236 
Selenite,  129 
Selenitic  cement,  135 
Selenium  in  glass,  312,  313 
Semi-automatic    bottle-making 

machines,  307 
Semi-opal,  39 
Sepiolite,  68 
Serpentine,  69 
Setting  of  Portland  cement,  159 

of  slaked  lime,  127 

ware,  208 

Sevres  porcelain  (new),  236 
Sgraffito,  199,  227 
Shale,  26,  29,  30,  108 
Shap  granite,  103 
Shaping,  190 
Shaw,  J.  B.,  329 

Dr.  Simeon,  181 
Sheet  glass,  303,  304,  318 
Sheet- iron  enamels,  328 
Shell  sands,  27 
Shepherd,  36,  36,  158,  259 
Shrinkage.     See  Contraction 
Siege,  290 


Siemens,  2,  290,  291 
Sieves,  186 
Silfrax,  117 
Silica,  3-45,  274,  282 

bricks,  33,  45,  259-266 

chemically  pure,  7 

for  glass,  282-285 

for  paints,  355 

(fused),  42V 

glass,  44 

threads,  43 
Silicate  cements,  166 

pigments,  350-357 
Silicates,  46-122 

classification  of,  50 
Silicic  acids,  50 
Silicified  wood,  26,  40 
Silicious  sandstone,  29 

sinter,  40 

tuff,  40 
Silicon,  1,2,  11,113,114 

anhydrous  protoxide,  4,  5 

carbide,  11 5- -11 8 

monoxide,  116 

oxycarbide,  119 

steel,  114 
Silicones,  5 
Sillimanite,  70,  71,  73,  209,  268,  300, 

311 

Siloxicon,  118 
Silundum,  119 
Silver  in  glass,  312 

ultramarine,  80 
Silverman,  A.,  315 
Silvery  irons,  113 
Sinopis,  352 
Skeleton  oven,  203 
Slag  cements,  111,  161,  162,  165 

fertilizers,  347,  348 
Slags,  109-111 
Slaked  lime,  127,  128 
Slate,  4,  17,  70,  108,  109 

black,  355 
Slip,  141 

blunger,  184 

kilns,  187 

painting,  226 
Slurry,  141 
Smalt,  356 
Smearing,  214,  233 
Smithsonite,  56 
Smoky  quartz,  18,  19 
Snake  stone,  31 
Soapstone,  68,  355 
Soda  alum,  341,  342 

felspar,  91,  92 

Soda-lime  glass,  279,  302-304 
Sodalite,  78,  79,  80,  95 
Sodium  alum,  341,  342 


INDEX 


373 


Sodium,  metasilicate,  57 

permutite,  97,  98,  99 

silicate,  6,  7,  42,  57 

silicate  soap,  60 

sulphate  for  glass,  285,  287 
Soft  porcelain  (French),'  235^ 
Soils,  26,  348-350 
Solid  solutions,  46  £ 
Soluble  anyhydrite,  130 

glass,  57,  280 

Spalling  of  silica  bricks,  260,  261 
Spanish  topaz,  19,  71 
Special  cements,  164 
Specks,  186,  209 
Spence,  339 

F.  M.  and  D.  D.,  341 

P.  and  F.  M.,  344 
Spengel,  A.,  98 
Spessartite,  90,  91 
Sphene,  79,  102,  104 
Spherulites,  106 
Spiller,  93 

Spilsby  sandstone,  30 
Spinel,  27 

Spodumene,  64,  100 
Spraying,  215 
Sprinkling,  213 
Squatting  temperature,  276 
Stack  ovens,  206 
Staffordshire  kiln,  253 
Stain  for  body,  185 
Staining  glass,  315 
Staley,  H.  F.,  327,  328,  329 
Staurolite  (Staurotide),  27,  72 
Stead,  J.  E.,  265 
Steatite,  67,  68,  74,  355 
Stein,  W.,  322 
Stein  furnace,  293,  294,  333 
Stein  and  Atkinson's  tank  furnace, 

301 

Stereochromy,  62 
Stilbite,  95 
Stokes,  147 
Stone,  artificial,  28 

grinding  of,  183 

yellow,  351 

Stones  in  glass,  286,  311 
Stoneware,  9,  178,  179,  230-233 
Strains  in  glass,  318,  319 
Strass,  321,  322 

metal,  309 
Stream  tin,  27 

Strengthened  plate  glass,  304 
Striae  in  glass,  318 
Striped  agate,  20 
Stucco,  134 
Sturtevant  open-door  ring  roll-mill, 

144 
Sulphur  in  glass,  312 

moulds,  193 


Sun-dried  bricks,  244 
Sunstone,  92 
Syenite,  104 
Syriam  garnets,  90 

TABASCHIR,  40 
Tabular  spar,  62 
Talc,  67,  68,  73,  314 
Tarn  o'  Shanter  hones,  32 
Tank  furnace,  301 
Tanks  (for  glass),  300-302 
Teisens'  gas-fired  lehr,  296 
Tempering  temperature,  276 
Tensile  strength,  175 
Terra-cotta,  178,  240,  241 
Terrar,  228 
Terre  verte,  353 
Thomas,  C.  W.,  300 

S.  G.,  347 
Thompson,  93 
Thomsonite,  95 
Throwing,  190,  191,  195 

wheel,  191 
Thulite,  89 
Tiger-eye,  20,  67 
Tiles,  77,  178,  241-243 
Tin  oxide,  228,  259,  314 

in  enamels,  323,  326-328,  331 
Tin  putty,  122 
Tinstone,  27 
Titanite,  79,  102 
Titanium  in  clays,  76 

oxide,  314 

Tobacco  pipes,  239,  240 
Tone,  115 

Topaz,  19,  27,  71,  100 
Tornebohm,  155 
Touchstone,  25 
Tourmaline,  27,  29,  49,  65,  87,  88, 

100,  104 
pincette,  88 
Trachyte,  32,  33,  105 
Trade  statistics,  167,  271,  272,  334 
Transparent  pearls,  46 
Trap  rocks,  4 

Trass,  124,  153,  160,  161,  163 
Travers,  Dr.  M.  W.,  286,  295,  299, 

300,  333 

Tremolite,  27,  65,  66 
Treuheit,  L.,  114 
Treussart,  General,  164 
Tricalcium  silicate,  55 
Tridymite,  6,  32,  33,  35,  36,   180, 

260,  261,  262 
Tripoli,  41,  121,  122,  134 
Tripolite,  41,  135 
Tschermak,  47,  48,  49 
Tschermak's  law,  49 
Tschermigite,  342 
Tscheuschner,  280 


374 


INDEX 


Tscheuschner  normal  formula,  280 
Tungstic  acid,  cloudiness  frem,  332 
Turkey  umber,  353 
Turner,  John,  226 

Prof.  W.  E.  S.,  277,  302,  307, 
319,  333 

W.  G.,  238 
Turner's  lathe,  192 
Turning,  192 

ULTRAMARINE,  42,  78-84,  356 

pigments,  355,  356 
Underglaze  colours,  228 

decoration,  224,  235 

printing,  226 

Up-draught  ovens,  201,  203 
Uranium  black,  229 

oxide  in  glass,  313 
Ure,  306 
Uwarowite,  90 

VANDYKE  brown,  354 
Vein  quartz,  18 
Velvet  brown,  354 
Venetian  red,  353 
Vernadsky,  51,  71,  72 
Veronese  earth,  353 
Vesuvianite,  100 
Vicat,  137 

Victoria  cement,  161 
Violet  ultramarine,  80 
Vitreous  silica,  3,  6,  36 
Vitrifying  clays,  206,  230,  232 
Vogel,  H.,  327 
Volatilization  (glazing),  214 
Volcanic  glass,  106 

rocks,  33,  65,  78,  89 

sands,  26 

tuff  (or  tufa),  105,  161 
Vondracek,  R.,  329 
Von  Lasaulx,  32 
Von  Rath,  32,  33 

WADMORE,  341 
Ward,  F.  O.,  92 
Washburn,  Dr.  E.  W.,  287 
Water  glass,  57,  59,  61,  62,  280 

cements,  60 

lime,  128 

-of- Ayr  stone,  31 

of  constitution,  49 

of  hydration,  49 

-opal,  38 

purification  by  alum,  340 

softening  by  permutite,  96 

stones,  30 

Watkin's  Heat  Recorders,  212 
Wax  moulds,  193 

opal,  39 
Weathering  of  baU  clay,  179,  180 


Weathering, protecting  stone  against. 

61 

Weber,  116,  280 
Websterite,  342 
Wedging  clay,  189 
Wedgwood,  J.,  181,  233 

pyrometer,  211 

ware,  233 
Wengers,  211 
West,  308 
Westinghouse,  116 
Wet  mixing,  185 

pan,  207 

Whetstone,  18,  31,  32 
Whirler,  195 
White  asbestos,  66 

garnet,  101 

glass,  279,  302,  314 

mica,  49,  85 

stone,  176 

sulphate  of  alumina,  344 

ultramarine,  80 
White,  S.  S.,  120 
White-burning  clays,  175 
Wilkinson,  A.  J.,  202 
Wilkinson's  down-draught  oven,  202 
Willemite,  56 
Williams,  W.  S.,  320 
Willow  pattern,  226 
Wilson,  G.  W.,  300 
Window  glass,  279 
Wired  glass,  204 
Witt,  J.  C.,  243 

O.  N.,  312 
Wohler,  5 

Wollastonite,  35,  50,  62-64 
Wood  opal,  40 
Wright,  157,  158 

J.  W.,  299 

YELLOW  ochre,  351,  352 

ultramarine,  80 
Yensen,  T.  D.,  321 
Young,  W.  W.,  266 

ZAFFRE,  356 

Zellner,  341 

Zeolites,  6,  27,  37,  73,  89,  94-96, 105, 

169,  349,  350 
Zernig,  269 
Zinc  distillation  retorts,  118 

oxide,  314 

sulphide  in  enamels,  330 

white,  62,  356 

Zircon,  27,  29,  79,  91,  102,  270 
Zirconia,  228,  268,  269,  314 

bricks,  258 

enamels,  331 

for  paint,  355 
Zirconium  carbide,  269 
Zoisite,  39 


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